There's really no issue in free jumping a Mustang. I bet she's jumped off higher cliffs then that as a youngster. As a 3 year old, there's nothing wrong with the height she's jumping (though I sincerely doubt it's 3'0", looks a lot more like 2'6" to me). The knees on a horse are closed by 2.5-3 years old. At this point, riding her would be worse for her then free jumping her because it's her spine that hasn't fused yet.
Gosh. I'm loving Yazi more and more everyday. I just can't WAIT to get her here and start working with her... :) She'll be coming down soon. :)