Can I use 2 two channel amps to run a 3 channel system? leaving one side of an amp unused

You should not sum up the left and right channels and feed it into one speaker.    The outputs from each channel on the amp are low impedance and there is a good chance they would "interact with each other" and short (or other bad things).   


You will need to use a stereo to mono converter.    This isolates the 2 channels and combines them to power one speaker.

Sure you can

As per mcreyn's suggestion, I contacted bel canto and they responded immediately. I was told "there should be no problem using one channel. Just don't drive that channel and no load needed."

Thanks to everyone for their feedback.