I've read that the mounting screws on b2 and b3 are placed different, so you cannot easily plug a b3 board in a b2 unit and vice versa.
Also, if you can run them 3 per unit, i think its preferred in terms of overclocking and power supply. You will need 1000W+ for 4 units.
Its a lot of amps running through a small control unit. But i will leave an engineer to answer that fully..