Hi,
Do you mean you're going to run the five speakers from three seperate amps?
It would be helpful to know what you are using as a source, ie a DVD player with DD/DTS decoders built in or is it perhaps a digital decoder/pre-amp? If so it is possible to connect the five analog out-puts of a suitable DVD player to the amps via co-ax leads (plus the .1 directly to an active sub), or if a decoder is used using an optical or co-ax lead from DVD to decoder.
If it's going to be straight from a DVD player to the amps you could be in for a lot of faffing around with the volume controls on the amps, due to the fact you'll have to turn all the amps up the same amount at the same time (unless the DVD player has a volume control that operates on the 5.1 outputs so it can control all three ams together). But seeing that this sort of arrangement is usually found on some high end gear and, no offence, by the sounds of it yours does'nt, otherwise you'ld be using KRELL or BRYSTON pre-power combinations, you may have problems.
Going back though to your original question :- yes it is possible to use another power amp to drive the centre channel, you need to get a Y splitter, connect the single end to DVD then connect to the amp with a stereo phono. Take the positive speaker feed from the left channel of amp, and the negative (or common) from the right and connect to the centre as normal. This way you won't be wasting a channel of amplification.
Get back in touch and we'll see if we can be of any more help.
TTFN