Could you possibly get an actual video of her moving, preferably from both side, the front and the back?
I can tell you that I would like to see more muscling on her rear, but she's young so that's to be expected. From the still shots, nothing really jumps out at me that is wrong with her. Her back is a tad bit long, but she is likely to grow into it.
With her being only three, she has anywhere from 2 to 3 years of growing left to do. Take your time training her, and ALWAYS use protective leg gear for her, preferably some that actually offer support. Like either Pro Choice or Classic Equine. Always keep in mind that she is young, and her bones are no where near knitted. I personally I don't like to start actual barrel training until the horse is a long 4 y/o or short 5. But there is plenty of training that you can do in the mean time.