At my barn there's a trainer who is working with a 3 year old (actually, I think he turned 4 this year) that is about half Percheron, one fourth Belgian, and one fourth Thoroughbred. She has sat on him several times, but compared it to "riding a baby elephant" and has gone back to doing ground work with him until he matures enough to carry his own body around and then hers. It is very misleading because he is so big (almost 18hh) and looks like a full grown horse, but his mentality and balance is still that of a young, growing horse.
As JDI said, do lots of desensitizing and make him the perfect gentleman for the farrier and vet.
When you do lunge him, try to do it on the largest circle possible. I like introducing an unbroke horse to side reins before I ride them, mostly for submission, but in this case, I would also lunge without them so that he can find his own balance and not learn to lean on the side reins for support.
"He doth nothing but talk of his horses."