I would say 5/8" is about normal for nock height for 3 under. 3/4" or 7/8" is not unusual. If you have no form problems that might be causing the high misses, you might want to try a higher nock height.
The only way I can tell if my nock point is right is to bare shaft. If you can't get a bare shaft to fly straight at any reasonable nock height, then you might have a tillering problem.
There could be something about your grip that makes you want to point high as you draw. Is it more of a straight grip and you're used to a more contoured grip that gives you some support in the heel area of your hand, maybe?