Are her feet bothering her? In my experience a lot of standardbreds suffer from sore feet, especially anything related to Artsplace. I find routine cortisone injections into their coffins works a treat. It's hard to tell exactly what you horse is doing wrong. Sometimes that three leggedness as you describe it can also be their hocks. Cedar Oil applied as a blister to their hocks (ask you vet where to put it) I also finds helps this.
I also have found rubbing liniment into their muscles daily helps with general stiffness/mild lameness. See thread
http://www.harnessracingforum.com/sh...and-needs-help
I normally try these sorts of treatments first before spending a lot of money with a vet, personally. A lot of the time they work and you save yourself $$$. If they don't, then you know you have done everything you can and its time to pull out the big guns so to speak
Good luck