I also shoot three under - I've noticed that while bareshaft tuning, finger pressure on the string hand would affect nock high/low position. Too much pressure on the index finger would result in knock high, too much on the ring finger caused a knock low. For me, even pressure on the index and middle finger, and less on the ring usually results in good arrow flight. I don't quite understand the physics behind it, but I think I remember someone here saying that when there's too much pressure on the index finger, when you release the arrow it almost drives down and bounces off the arrow rest, causing a nock high. Does that ring a bell?