Well, considering that sub is rated for 600w RMS, and assuming you have at least two of the amps which are identicle.. I'd say ditch that third amp, and just use two. One on each voice coil.
Just make sure to take great care that you tune both amps so they're outputing the same signal power to each of the voice coils.
This will give you the 600wrms the sub is rated for, and will keep your amps running cool at 4 ohms. Alternativly, if they're decent amps, you could parallel the sub to 2ohms running on One amp, and probably get about the same power as you would from both amps running.. this is the better option, imo.