Harmonically it's inadvisable, as "bass is boss" - the bass line provides the foundation for the whole harmonic context of the piece. Also, I don't know what pieces you're looking at or what style you're singing in, but it's rare in the common practice period for the alto to simply repeat the bass notes.

What is your knowledge of music theory and harmonic structure? It IS possible to drop parts, but usually you're going to have to drop either the alto or the tenor - rarely, if ever, the bass or soprano (the outer voices are the most audible - and as I said, the bass provides the foundation, while the soprano could be thought of as the roof). Usually it seems like a piece gives the more crucial harmonic parts more to one line or the other - you have to analyze each piece to see whether the alto or the tenor is more crucial.

For example, in Pitoni's Cantate Domino, the alto tends to hang out on the fifth of the chords in the harmony, or double the root of the chords. For that reason, in this particular piece, if I have to drop a voice I'd drop the alto.

In Lotti's Regina Caeli (at least in this edition: http://www.pucpr.edu/diocesis/cantoral/RegCoeliLotti.pdf), I reversed the alto and tenor parts (the tenor is extremely high and my lone tenor is not confident) and allowed the "alto" line to drop out.

It can be done, but you need a good knowledge of harmony and singers who aren't freaked out by singing different parts as needed.

From the outset, let me say: Generally speaking, it is unwise or impractical from a musical standpoint to make a habit of trying to sing four-part choral music with only three voices. No matter how one does this, it amounts to a rearranging of the composer's intentions, more often than not, leaving much to be desired. These notes speak to this issue and the problems it raises.

Already alluded to has been the problems of proper voice leading when trying to compress four parts down into three parts in such a way as to preserve as much of the harmonic integrity of the work as possible. Preserving harmonic integrity presents its own significant problems, if only because four-part harmony, compared with three-part harmony, is structurally quite different (some would say organically different). The combination of voice-leading and harmonic structure are intimately bound together into a package known as counterpoint. And four-part counterpoint is simply not the same as three-point counterpoint.

Counterpoint - not even that of such Renaissance masters as Palestrina, Victoria, Guerrero, Tallis, Byrd, Morley, Josquin, Mouton, Crecquillon, and many others (including those of the early Baroque era) - is not vertically (ie., harmonically) made up of a progression of "chords" which are pure in the sense that all notes are parts of a diatonic triad (whether major or minor) - nor does counterpoint consist norizontally of a series of "chords" (harmonic motion) upon which a melody rides (contrary to what those who say that all you need to know is I, V, IV chords at first and maybe a few II, V7, and VI chords later on). And, certainly, the mere fact that a note might be doubled (perhaps at the octave or super-octave) in two (occasionally three) parts does not mean that one of the occurrences can be removed without changing the resulting sound. As for situations when all four notes are different, there are countless carefully crafted "dissonances" arising in all sorts of situations, whether in passing notes and suspensions or in artfully designed moments of tension. The judicial use of major 7th/minor 6th and minor 7th/major 6th harmony, as well as 9/8, 4/3, and other suspensions, even when used only in passing, make the great Renaissance and Baroque works shine aurally in ways that three-part harmony cannot convey, especially when these are juxtaposed with moments of resonating consonance.

This is not to say that three-part (or even two-part) harmony does not have its own myriad possibilities. The relative paucity of three-part works compared with those for four or more voices (occasionally significantly more) probably speaks as much to the richness and variety of textures available to Renaissance composers and singers as it does to the ready availability of actual singers themselves in sufficient numbers to perform such works - the latter issue of availability of singers being a significant obstacle for current performance in all but a few venues such as cathedrals and large metropolitan churches. On the other hand, the necessity for church musicians to seek out three-part works to sing has made it possible for present day composers to explore new possibilities in serious sacred three-part a cappella choral writing that open new vistas, even as they hearken back (as it were, "ad orientam") and reestablish ties with the sacred musical tradition of the church. Probably no better exponent of this new tradition in three-part writing is Kevin Allen, who has been teaching all of us (even me, in my old age) by most worthy example.

To return to the issue of rearranging four-part choral music to three-parts, one must also pay attention to the types of voices for which the original work was written. As sacred music moved from the cathedrals and academic chapels of the Renaissance, where choirs of men and boys were generally predominant, physically into churches and temporally into the modern practice of mixed choirs of male and female voices (think mixed SATB choir, with SA women, TB men), choral music took on new sonic dimensions: high and low women's voices mixed with high and low men's voices. This became a substantial, if not predominant, part of the musical idiom for hymn and choral writing as well as for four-part SATB motet writing through the Baroque, Classical and Romantic eras into the 20th Century and Modern eras. Any attempt to "compress" such four-part mixed chorus writing into a three-part texture - such as SAT, SABar, or STB - has to give up something, for it upsets the balance and interplay between women's and men's voices.

There may be rare exceptions to the compressibility of four-part music - as, for instance, when a basically three-part work has a fourth part that is in the nature of an optional "descant" - but most of these exceptions will be in situations where the original work is purely conceived as a four-part work, accompanied, with an arrangement contrived for which the accompaniment provides (at least some indication of) the missing voices. Even then, significant rearranging might be required, with less than ideal results. Here is a case in point:

In my own arrangement of the Mozart Ave verum corpus for three voices - an accompanied work - the effort has been made to preserve the alternating two-part high and low dialogue between SA and TB voices in the "Esto nobis" section by having the middle voice flip back and forth between the A and T parts. Thanks to the work being accompanied, the result works - but, in my estimation, just barely. All in all, the compromises that I had to make in this arrangement are less than gratifying but might satisfy those who simply must perform the work with only three voices. My recommendation: if you can find a way to do the SATB original, then do it in preference to the SAB arrangement.

For unaccompanied (a cappella) four-part works, the problems are much, much worse. I seriously doubt the possibility of arranging in an acceptable way, without the result sounding woefully lacking, any of the following venerable four-part works:

Palestrina: A solis ortus cardine; Sicut cervus
Victoria: O magnum mysterium
Guerrero: Ecce nunc tempus
Josquin: Ave Maria .. Virgo Serena (in the Colloquium XXII repertoire)
Mouton: Quaeramus cum pastoribus
Crecquillon: Sancta Maria
Tallis: If ye love me
Byrd: Ave verum corpus
Morley: Agnus Dei
Purcell: Thou knowest Lord the secrets of our hearts
Bruckner: Locus iste

All of these works are available free from CPDL.

"... the bass part was an exact copy of the soprano."

How dreadful. It's quite different from, say, the shape-note practice of "burying" the melody in the tenor.

3-part vocal music requires a special craft (hey, even 2-part and 3-part instrumental music requires a special craft, but all the more when everything's within the relatively narrow compass of amateur human voices). Byrd could do it, supernally. Many others, not so much, shall we say.

For Catholic sacred music, the shape and counterpoint of each vocal line is more fundamental than vertical harmony.

CHG

My reference to vertical harmony was referring to the idea of lining up chords vertically under each note. In both ideal and practice, that is subordinated to the lines - to the extent that one will often find moments that are ambiguous in harmonic terms. I am not saying deeply subordinated, but it is subordinated. The difference I am drawing is the difference between a singer's perspective and a keyboardist's perspective. The keyboardist can be tempted into being concerned about what he or she is doing at any given moment, vertically, simultaneously. The singer is more concerned about the drive of the line, as it were. And the singer has more of a bead on how the Church's sacred music works.

If you've got three people, I guess you want to find music written for three people. And it's certainly out there. It would narrow your choices somewhat; but that would make it easier to choose a piece, probably.

CHG

Agreed. (I've just met keyboardists who don't think that way instinctively, but have a different conception about which is the warp and which is the weft. Some of them write music, too. In fact, this is one of the more common problems with contemporary worship music: you can tell that it is written by some who thinks more vertically than horizontally, as it were.)

I have a tendency to agree with Musicteacher56 on this one. Practically, most of us need to do the best with what we have.

For example, I frequently perform selections from Rice's Simple Choral Gradual as just SAB, with the organ lightly accompanying while filling in the tenor line. I COULD sing the tenor line myself, while playing the organ, and trying to conduct, but I think that would just distract my baritones.

That said, there is a great deal of music available for 3-voices on this site within the "Secunda" anthology. I also have a .PDF of a different anthology of 1, 2 , 3, and 4-voice motets that I would be willing to send to any interested parties.

To recap, it is not ideal to sing even moderately complex polyphony with fewer parts than the composer indicated. In the trenches of parish music-making, it might be acceptable if forces dictate. Each piece will be different and YMMV.

"adapting 4 part psalm tones into 3 parts"

I think that would probably work just fine. On the off chance that something really interesting harmonically takes place in the omitted part, you could just play that by ear.

I'm curious, Chris, with which psalm tones are you working?

I think this sounds very good sung with only the first two or three parts at the top, (alto, soprano, tenor I presume) as well as good with all 5 parts.

Magnificat - (Tone I) (1973 ICEL text with Fauxbourdons by William Byrd) (16th c.)

Perhaps someone else can make use of this besides me. Hmm. I'll create a new post for it perhaps. I am going to typeset all the other 7 modes into the same magnificat text with the 16th c. fauxbordons eventually too. It will take a few months to finish it up.

There is another reason to sing four part music with three singers. If you don't, that lone tenor you need who has left his own parish to avoid crummy music won't hear you sing and come up to the choir loft afterwards, saying, "I know that part...."

Prolly so, Noel.

On her dissertation it says "Emilie Julia Wingo Shinnick, B.S., M.A., B.M., M.M." perhaps she prefers to be called Julia the rest of the time?

I am enjoying her dissertation it has a transcription for a sequence for St. Christopher "Xpisti calix quam preclaris" from the Manuscript Assissi, Biblioteca del Sacro Convento, MS. 695. It also has a version of the "Christus Vincit" with more saints and more elaborate melody than that in the Liber Usualis.

You can get arrangements for 3 equal voices. I have a copy of Carmina Sacra: 17 Motets for 3 Equal voices which was published in the 1940's and has been out of print for decades. I was lucky to be gifted with one from the Seminary of the Diocese of Melbourne.

Kevin Allen's book "Motecta Trium Vocum" (12 motets for 3 equal voices) is also a great starting point. He has even suggested transpositions for mixed choirs (4 voices to sing a 3-part motet). There are provided a number of practice recordings on the internet, which including separate ones where one voice stands out from the other 2 in order to aid instruction for those who don't read music all that well. It is the standard book of motets for the Seminary of the Diocese of Sydney.