Colin,
I have a bit of personal experience in this matter - my A/A TB was hotter than a pistol, hot/explosive/dangerous on a daily basis. What follows is a compilation of things I learned while (honest!) trying to save this horse from a dogfood can.
First thing to look at is lifestyle. Horses are designed to graze 18 hours a day, perhaps covering 30 miles in a day looking for food and water. When we put them in stalls, we stress them to some extent (some horses more than others). In my experience, less than 8 hours of turnout a day leads to energy issues, vices, colics, ulcers, and a general lack of practical muscle tone and fitness. My TB lives out 24/7 because having a roof over his head makes him just WILD (even today). Out in the pasture, he kicks back and plays with his mule friend and eats. He keeps himself really quiet and happy.
Secondly, what are you feeding? If you’re feeding this young horse quite a bit of food, that can easily cause A.D.D.-like behavior. We’ve gone to higher fat, lower carbohydrate diets for all our horses, and they look great and are quiet. Each horse’s body chemistry is different. My horse eats lots of Purina Strategy and is still quiet as a mouse. I’ve seen other horses go wacky on Strategy. You need to find what works for this horse. As a general rule, it’s best to feed the least you can of concentrates and focus on pasture and hay as the main source of sustanance.
Thirdly, and this may or may not apply to you, but I’m going to throw it out because it was a huge part of my horse’s problem. How do you guys get along? My horse spent all his time trying to run away from me, whether I was trying to catch him or if I was riding him. He was always running out from underneath me, and felt “bound up” like a spring waiting to blow. He hated me, he hated being ridden by me, and for a long time I treated it like a training problem. It was a problem between the two of us. I needed to be his friend FIRST, his “owner” or “trainer” second. Once we got this going, things really started to turn around, and today, he’d do anything for me, just because I asked.
Additionally, remember that 3 is still very young. The bones in a horse’s back and hips don’t fully solidify until he’s 5 or so (long after the knees have closed), so I’d avoid fast/strenuous work if he were mine, for the sake of longevity. There’s lots of training you can (and should) do that doesn’t involve going fast, that can also work to keep a youngster’s mind engaged - turning, stopping, backing, lateral work, opening gates, trail riding, softening the jaw/poll, etc. This stuff, established well and early, can serve a horse for the rest of his life.
I think sometimes, we give our horses plenty of physical work, but neglect to give them enough mental work to keep them engaged. After all, do they REALLY care about this riding stuff? When a hot horse’s mind is put to a very specific task ("I’d like you to walk at this speed, in this direction, with your head here and your hips here), I’ve found that it really engages them. As time goes on, instead of adding faster work, you can get more and more specific - there are so many layers there to peel off!
Just my experience…