I think a 3 yr old should be limited to jumping only with a professional so that it is only done for the horses benefit, not just for fun. Also, you want a solid foundation in jumping and this is the ground floor. I'm not saying you aren't capable of jumping the horse or are doing anything wrong in its training, but a horse that young should only be jumped lightly, and if at all possible by someone who will truly improve the horses ability over jumps. It's hard to explain exactly what I mean, but with a young horse you should limit a session to as few jumps as it takes to teach the lesson, and a professional is best suited to this. If you make a mistake not only will you have to correct the problem, but it will probably take more jumps to establish the right way of doing things. If there is no trainer present and you are the horses only way of introduction to the job that will be required, then be careful to limit your jumping to only what is neccesary for experience purposes and not jump just for fun at this age, there will be plenty of time for that when your horse is older. Otherwise low fences at this age are acceptable, but I wouldn't start until about half way through their three year old year.
Troubled TB ~"A thorn by any other name will prick just as deep." @-'--,---