Try the Byrd Mass for Three Voices. It is really ATB and usually sung down a step for that combination. I have done it SAB at pitch and that is quite satisfactory. When I sing it with my full choir, I split the tenors, some singing the A part some the B.

The piece is a wonder of counterpoint, making the very most of the limited resources of three parts: you cannot double any note of the triad, and so every note has to count; maintaining full triadic sonority involves some real tricks, all of which Byrd has developed to the full.