A manufacturer's motor voltage recommendation is more of a recommendation than an absolute maximum and in any case, a 6S Lipo pack will drop to about 22.2V (3.7V per cell) under load.

Like Mario said, 30A is a very high current for the motors to be drawing under no load - there must be a lot of friction in the drive system to require 720 watts of power just to spin the wheels.

Also: three motors for drive, what's that all about?