Like everyones said, wait until 4.
We don't train our horses past cross rails and 2 feet at 4 either. I know lots of people will jump a lot higher sooner, but there a LOT of things you can do in jumping training that doesn't need higher jumps. Things like changes, pace, regularity, confindance and responce can all be taught without jumping over 2 feet. It also gives the horse a very safe amount of time to grow up, and GREATLY reduces the chance of the horse developing unsoundnesses like arthritus as it ages. We focus on basic flat at 3, latteral movements, leads, etc and basics to jumping at 4, and then they're ready to really ride at 5. Once the horse is 5, we know it's bones are physcally mature, and we'll start on higher jumps. We find that since we took the time as 4 year olds to get the basics perfected, height doesn't scare them, and they can enjoy their job more because they learned how to do it so well at an easy level.
I would STRONGLY recomend teaching your horse latteral movements before even consider jumping. We don't start our horses on anything past ground poles until they recognise the signals and pick up correct leads. And that comes from good schooling on the flat. It can help a lot when jumping. If your horse doesn't know latteral aids, it will more likely take to running out when confronted with a test than a horse that you can keep straight all the way to the jump with your leg.
I hope this helps, good luck.
EDIT: A little story about jumping horses too fast.
There's a barn in town here that's natorious for jumping horses to young and pushing them too high. At one show, they actually took a 4 year in a 5-bar-challenge (5 jumps in a row, keep raising the jumps until they knock them down). Unfortuanately, the 4 year old won (highest was 3'9, it was a small show, lol), making the owner think what they did was right. But the truth is, is that they had been jumping him all day at 2'9 - 3'0, jumpers with tight turns and tricky lines. The poor horse was so scared, and when his rider pulled out the whip, he shut down. He wouldn't move, and then he would rear before giving into her and finishing the course. He was afraid, and they hadn't taken the time to school him. The straight line of jumps was a relief to him. After he went down it twice, he grew a smile on his face and just did it, because he is SUCH a good horse, and FINALLY understood what he was being asked to do! The story does have a happy ending though. 2 weeks later a good friend of mine bought him. He had 2 huge splints from the show, that have gone down a lot since, thank goodness. The first time she rode him he was SUPER tense and was trying to rear and bolt. She simply made him walk, and when she did, she left him alone, he hasn't reared or put a foot wrong since. They're spending the next year focusing on dressage and very basic jumping, since he was scared so badly. The first time we did a cross rail he got a bit hot, but we just trotted it afew times, and now he completely trusts my friend isn't going to push him too fast, and he's doing everything she asks of him, as if he knows that this girl saved him.