Have you tried the CPDL Multi-Category Search?

There are 47 sacred, a cappella works for ATB choir (33 in Latin).

Most late-medieval/early Renaissance music a 3 should work. You may have to fiddle a bit with pitch levels to get into the sweet spot for your folks. In my experience, it's a novel position to be in.

Here's one I am preparing this morning for the Choirbook series.

Only 15 measures, but don't let that hold you back from singing it. After singing the Communio, the organist could play it through once on a simple 8' flute stop, then one singer could sing it, accompanied, then three singers could sing it with the entire schola or choir entering on the Miserere followed by the entire choir singing it once again unaccompanied, crescendoing gradually through the O Bone Jesu section, reaching a very strong Forte on the final Jesu! Make a clean cut off, do a grand pause to let the sound die away and then sing the last Miserere section pianissimo.

I've dropped it a whole step to make it comfortable for ATB in the "down" version.

You could also try Small Choirs International.

The Mass for Three Voices by William Byrd is just for these voices. It is moreover a real masterpiece.

Virtually all polyphony can be done at a lower transposition.

Thanks, Chris, but we don't have enough singers to cover it.

Thanks, everybody, for your help. I've got several good leads to go on!

Thanks so much. I've got a new motet I can share, too, if needed.

The Seventh Chester Book of Motets: Motets for 3 voices, ed. Anthony G. Petti, has proven invaluable for ATB groups. Some of it is SSA that works splendidly down the octave (or thereabouts).

We have quite a lot of ATB music built up over the last 30-some years at Immaculate Conception in Cleveland. I have a list of pieces but I am struggling to update it so that it reflects the present situation. We have a lot of ordinaries as well as motets for many Sundays of the church year. There are a lot of contrafacta that I and others have done from pre-existing pieces and textless repertoire from the Middle Ages and Renaissance. If you have specific needs, drop me an email.

Fred