It's actually very common practice, part of the reason many hunter and jumpers are put on joint medicine and get their legs injected. Just like when people start there horses as yearling it's to be competitive in the show ring at 5 and 6. I'm for 3' at 4 year old, as I don't think that's a substantially high jump, but 3 years old it's a little early.
Full horse bone fusion in most breeds is done by by 6 (younger for most pony breed, some are typically done by 4), with the last of them being the spinal column at 5 1/2 for mares and 6 for males. By 4 I think horses are ready for full work, because their legs are done, including the hock, which is one of the last bones. I'm not saying they are ready for grand prix jumping or advanced dressage trainer, but I think they are fine to jump 3' or so (although, jump height will differ depending on the horse. My trainer's 3 year old horse "jumps" 2'6" in that he takes a big canter stride over it. He's 16.3 hh)
By 3 a lot of important bones are not finished, and can cause very bad, long term damage, which may not be apparent immediately. The humerus, tibia, and femur would just be fusing now, and the pelvis and hocks won't even start until late into the 3rd to be finished by about 4.