If this is for multi-channel audio (home theater etc) then I'd use the front pair by themselves before adding in a totally unmatched center speaker. It won't sound right for the center and main speakers to have such different sound quality.
The proper way to run a center channel is to feed it audio that was deliberately mixed for the center channel, such as from a Blu-ray. You can also get acceptable results by running 2-channel audio through a processor that creates a matched center-channel track (such as through a receiver). You can't just feed a third speaker the same signal as one or both main speakers, though. If you feed it the audio from one side only, the phase and timing will be off through a combination of constructive and destructive interference, the relative output from one side will be markedly different from the other, etc. If you try to feed two separate channels into each other, they will interfere and not produce anything you want to hear.