Thanks for the replies.
"Power = Voltage^2 / R so it depends upon the impedance (R) of each speaker. Note the impedance can vary a lot over frequency, so you may end up with strange sounds. You'd also have to figure out a design to keep the impedance at the amp high enough when driving three pairs of speakres. Not all that hard, but potentially wastes power, and your question indicates circuit design may be a stretch."
I forgot to mention that the speakers will all be 8 ohm units. Thanks for the info on how the impedence might vary, thats pretty much what I was looking for. I found a unit that is rated to +/- 2 dB across the spectrum which I could use for the main speakers, but it just seems like inserting an unneceassry component in the chain at the end of the day. As far as designing the circuit I was thinking something along the lines of a switchbox or volume control with impedence built in (since I need volume control anyway), so I wouldn't have to really figure it out myself, just configure it according to the manuals, sorry if that wasn't clear.
HAMP568, my reasoning was mainly trying to avoid having to pick up a 7.1 reciever when I might get away with a stereo since my system is already a good amount over budget... that and if im installing volume controls anyway the impedence matching ones wouldn't be extra work.
Now I am seeing some decent prices on 7.1 systems so I will either go with that or do 5.1 with the impedence volume controls on the second and third rooms. One related question I have is that if I am setting up a 5.1 system is there any disadvantage in connecting my good speakers to the surround channels (Most models seem to warn against connecting multiple speakers to any channel besides the front for some reason)