That would be a nice project. It's not easy but nice. If I have the privilege to do that project I'll do a 5 Channel up to 7 Channel system then just buy my self a Sub like the Axiom The Epicenter EP600 V2 or 2 of the EP400 to make 7.2 Speaker System.
For 5 Channels - 5 speakers for front and rear and 6 speakers for the center channel.
For 7 Channels - 4 speakers for Center, 5 speakers for Front, and 3 speakers for Rear and Sur-Rear.
Depending on thier Impedance the 3 speakers is the most dificult convert unless you do your own modification and desgn a proper distribution unit so It'll holds its impedance.
Example if all speakers have the same Impedance.
for 5 speaker arrangement = 3+2 will make you closer to its impedance. (3 speakers on series parallel with 2 speakers on series)
for 4 arrangement ofcorse is 2+2 will make it even. (2 speakers in series parallel with 2 speakers in series)
for 3 arrangement is 2+1. (2 speakers in series parallel with 1 speaker)
But if you get a proper distribution unit for all the speakers then its better than doing the series and parallel connection.
Then the Onkyo TX-NR906 Amplifier
http/www.onkyousa.com/model.cfm?m=TX-NR906&class=Receiver&p=s