Ok - based on what I could see on that HobbyKing product listing, that chassis has the 1:34 motors, which are detailed with specs at Pololu:
The motors are 6 volt motors with a stall current of 6 amps.
So - for three motors (per side) - that would be 18 amps of stall current. Here's what I would do:
Rather than purchasing an h-bridge driver setup for each side, instead purchase a couple of these:
Then run them off of 5-cell NiMH R/C packs (6 volts) - these may have to be custom made; you could also try some kind of LiFePo or LiPoly packs (though the latter will be an issue because a 2 cell pack will output over 7 volts - which will cause the motors to wear out much faster).
Using a pair of ESCs rather than an h-bridges means you can easily control the speed of the motors using the Servo library, plus such ESCs will likely be a cheaper option. Something to think about, anyhow...