
Are you training at home in the presence of the two other dogs? I can't quite understand what you are saying regarding him concentrating at shows with the other two dogs. Are you taking them out of the car together, if the other two are left in the car while you work this one I can't see how he concentrates on the other two.
Depending on your answers to the above I would
1 Take out all three to a training area (park) and work them individually.
2 Take only your problem dog to a show, could even be a companion show, don't enter him just work him beside the rings within the atmosphere of the showground.
It sounds to me that he might be a bit stressed out by the occasion and is looking to your other dogs for reassurance. Treat him gently and build his confidence slowly.
Sandra