'Doff of the hood' to Arlene Oost-Zinner at Novus Motus Liturgicus who has an article on Kevin Allen's Motecta Trium Vocum. These are a set of twelve Latin motets for three voices. The disposition of the voices may be varied according to what you have available and considerable support material is available to assist in learning the motets. A careful selection of texts means that a motet can be found appropriate throughout the year and 'extension' chant passages are provided so that the length of the piece may be adapted to the liturgical needs. The NLM link will lead you to sample recordings (all parts sung by Matthew Curtis through the miracle of multi tracking) and an introductory video. It looks like something that could be very useful and a worthwhile investment. The compositional style is very approachable. Kevin Allen's link is in the composers list on the right of this page.