As a matter if fact i do run a 4 channel like that. In my truck i run channels 1&2 on my fronts and 3&4 bridged to a sub. But i guess im ghetto in my 79 pickup with 9" of lift and 35" tires. Whatever. I didnt tell him to run 3 channels on the sub either.because that is what al;l ghetto ppl do. buy a 4ch amp to run subs. do you have a 4 ch running your subs? if it is such a good idea then you should do it also. it is not as efficient of an amp as a class D either and that's just the start of why it is stupid. i wont reply here anymore you help him run his sub off 3 channels and let me know how it works out.
Back to the original question...
you can run 2the channels to your fronts. In my setups it was louder and sounded better, but i used a crossover too. As far as the sub, as its been said you cant bridge two channels on your sub, because your amp isnt stable bridged at 2 ohms. So you a can only put 1 channel on your sub, which will work but is NOT optimal by any means.
My advise to you is maybe sell the four channel to get funds to get a mono.