Not sure if this would be a terrible idea or not, but you might check out Feroci's 3-part "arrangement" of Victoria's O Magnum Mysterium, and have the treble voices read the tenor clefs as treble clefs.
The Christmas version of my setting of Tollite portas is very simple and based on the text given in the Offertoriale.
The late medieval Christmas hymn "There comes a galley laden" is very popular in Germany, though less so in English speaking countries, I think. Here is a three part setting. The text origin goes back at least to the 14th century, and, although the melody today sung is later (17th century), it has quite an "antique" flavor which probably is responsible for its popularity. Note that the unusual switch from perfect time to imperfect time is a marvelous musical expression of God becoming man.
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