Do subwoofers have inputs for the two main speakers and then you would run the speakers from the sub? If thats correct then is there a crossover in the sub so your woofer and tweeter only get the higher frequencies?
If it's a passive sub which requires speaker wire to be run to an amplifier, you will need an amp capable of driving a hard load, I wouldn't recommend running speakers from the same unit.
If it's a powered sub run the sub from the left/right pre-outs of the amp into the left/right filtered inputs of the sub using 75 ohm coax cable. Make sure you set the crossover on the back of the sub. (80Hz is THX standard)