Receiver options.
df4801,
While there are several methods for achieving your goal (distribution amps, speaker selectors, volume controls, etc.) I think that your assumption about the Yamaha 5760 and how it would work are valid.
The Yamaha 5760 is a 7.1 channel receiver and has a 7 channel stereo mode. It is my understanding that this mode sends the same information to each of the left and right channels (i.e. Left main is the same as the Left surround, etc.) Therefore, 3 pairs of speakers could be connected to the mains, surrounds, and rear surround channels running 7-channel stereo mode.
Also, TV or other speakers could be connected to the Speaker B terminals if not used at the same time as Speaker A. Since TV's have their own amplifiers, a line level connection from the Tape Output of the receiver might be more appropriate for the TV, in which case all speakers could be on at the same time.
The speaker selectors mentioned in the post above will devide the power from 2 of Yamaha's amps between 2 or 4 pairs of speakers. Thus, you would have <100 Watts / 4 = <25 Watts per speaker. If you connect each speaker to a seperate amp, then you have the capability of playing at higher volumes in the 3 rooms.
You should be able to get a good deal on that Yamaha with the newer models out.