I also have an a cappella group of three, 2f 1m. and a double quartet of about 8 of us. Some of the early stuff we do is in three parts. Pastime with good company, Henry VIII, Blow thy horne hunter, Thomas Whytehorne, Gaudete, anon. Some rounds are also in three parts. Oh and catches are often in three parts too, ie Cakes and Ale, Purcell. I would have to think to get any more. We also do a lot of English folky stuff just freely harmonized, it works for us. Good luck. JohnB