Does anyone have any recomendations how to synchronize (3) Tetrix motors, with encoders, in the same PID loop?

Unfortunately, it is not currently possible for ROBOTC to synch TETRIX motors together; for more information on why this is not possible, please see this thread. You could, however, possibly write a custom PID control program that would essentially synchronize the motors.

You can write simple PID control yourself to do it. However, if latency is an issue with Tetrix motors, it may not address the issue. But in theory, if you have your own PID control, you probably can tune your PID constants such that the system doesn't oscillate. If the system doesn't oscillate, the latency is less of an issue.

A basic position loop comes to mind where the position of each motor is compared and the difference between them (error) is used to increase/decrease power to the others. I'm thinking it would be best to declare one of them as a master and other two as slaves. The only exception would be if the correction power exceeds 100%, in which case I would have to slow down the master.

Does this sound like a reasonable approach?

Can you recommend a good resource to provide some PID background?

Just google PID control. Wikipedia has a pretty good description of it.