OTTBs are awesome, and sounds like you're doing great with him so far! As far as his biting, I don't know how long he's been with you, but if he's fairly new he's probably overwhelmed with not having to spend his life in a **** stall and maybe that's why he's a little "overzealous" with your other two. If he's driving them nuts, I hope you can put him in separate area. Hopefully if he's not been with you too long he'll start to calm down. I'm sure he's never had a normal social life.
What if you do everything on his right? Mount, tack up, and ride like you are already doing. Everything on the right, deprogramming him, and once he's catching on which I'm sure he will under your guidance, little jigs to the left, then back to the right all the time, etc.
Another idea, if you have a very, very small area to work in initially and can gradually expand the area bit by bit as you think he is gaining the understanding, the built-in limitations of space will help him pick up quickly what you're telling him. If he wants to break, you can just go back a step to a smaller pen.
Another help may be to have someone pony you two at a snail pace lol and get him used to just donk donk donk on the trail.
The problem you may run into later is that once you get him deprogrammed, putting him into more than a walk is going to be the next step, but if you use the same methods I think he'll pick it up super fast.
I think the world of you for taking on an OTTB :) Big kudos!