Your nocks need to be a little tighter to shoot 3 under than they do for split. However, they should still be just tight enough so they don't fall off the string once they are broken in.
Everyone has some degree of relaxation of back tension when they release the arrow. The best shots are the ones with the least amount of relaxation of back tension until the arrow is clear of the bow. You can improve this by using a Formaster, or just by being aware of it, so you know which shots had good back tension, and which ones were sloppy. If they get too sloppy, and you start collapsing before the release happens, then you should really let down the shot at that point. If you don't, and don't manage to recover back tension, that's when you pluck the shot. If you don't get a crisp release because you're just beginning to lose a little back tension during your release, but don't go into a full blown collapse, that's when you can get a high flyer or other kinds of missed shots.
Your elbow needs to be where the force of drawing the bow is directly in line with the arrow. Sometimes, when viewed from the side, the elbow appears to be a little high when it's in the correct position. You should be able to become aware of the feeling when it's in the correct place. However, even then there will be times when you lose concentration and don't pay any attention to where your elbow is. Your elbow may droop when you get a little tired or have shot a lot of shots. After I shoot a high flyer, in the next shot, I make sure my elbow is up where it should be and that I'm maintaining as much back tension as I'm capable of through the conclusion of the shot.