This arrangement of the Eis Polla trio by Bortniansky is sung by a small group of women (SSA) or a small group of men (TTB) in three parts, then concludes with a larger Eis Polla in SATB. In our parish we will do the SSA arrangement - soprano, second soprano, and alto. At the last SATB verse, the Sopranos will continue to sing the top soprano line; the second sopranos will switch to the alto line, and the alto will switch to the tenor line. I will sing a descant on the final repetition of Eis Polla as well (not recorded).
I've attached a video for reference, audio files for each part (they're not totally polished but should still be helpful for rehearsal) and the sheet music. Note that I have transposed the music down a step in my audio recordings.
This trio is sung after "O Come Let Us Worship" from the centre of the nave.