Im sure there are some kind of supplements out there, but I wouldnt do any growth supplements because it could distort his growth plates now, or affect him during an older age. Just allow him to grow and give it some time, I wouldnt sell him because of his height. We breed and raise alot of foals and they are always changing and growing, and once he reachs about 5 I would then make the decision based on his height whether you keep or sell him, but for what you would like to do, I wouldnt worry about his height our yours. Smaller horses can move just as quick as those big ones!! I prefer shorter anyway hehe. My bf is 6'1 and rides all my little reining horses and actually doesnt look giant on them, but he prefers the littler horses over the bigger ones too.
So really its up to you, but I would wait it out, esp if he is a level headed 3 year old, those are good keepers!