rule of thumb I use for car wiring is that a #10 wire should be protected by a 30 amp fuse, a #12 with a 20 amp, #14 with a 15 amp, etc. It sounds like you are going to need about a 30 amp main fuse/breaker to keep from popping it at full stall (if you can actually get the robot into such a situation), so maybe a #10 wire for the main power would be appropriate. Wiring for the individual motors would probably be #16 with a 10 amp fuse/breaker for each, and you could go smaller on the power feed wiring for the controller.
Hopefully Al will chime in too
(edit: Richard, he said the SUM of the three motors is 27 amps, not 27 amps each)