the rear speakers arent coaxial anyway. you can put components back there, but depending on your ride you may find no one makes one that size. I have never seen a 4 inch component set, which is what the e36 uses. If the components are rated at 100W RMS, then you should at least give them 75W RMS. If 100W is the top, and there is a manufacturer specidfied range, then 50 may be ok. As a rule of thumb you want your RMW to be at least in the top third. Power is distrubuted equally among all channels. No mix and match, unless you bridge, which wont do aything since you'll have to wire in parallel and negate any gain.
The three ohm thing is new to me.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>