One of the most important parts of a tremolo is uniformity of tone. All your fingers, a,m,i, should be uniform in tone and volume. If this doesn't occur, you get a gallop and lose the smoothness and continuity. A good exercise for this is to practice a, then m, then i, but very slowly and deliberately; utilizing planting, with the goal of uniformity of tone and volume with each finger. Add the thumb if you want, but the concentration, a least for this exercise, is on a,m,i. And please, don't worry about what each segment of your finger is doing, or what the knuckle is doing etc. etc. If your basic free stroke mechanics are in, then it will all work out. It's getting the right feel. Don't even look at the fingers of your right hand. (OK, you can look once in a while). Also, go for a little bit more volume when practicing this. Just short of banging.