Why not use 3 pairs of speakers and an impedance matching speakers selector box? This way, you only need one stereo amp, and don't have to worry about "Y" adapters.
"Y" Connector: Can Split A Signal How Many Times?
.
Your preamp has pre-outs for amps for two speakers, you use two "Y" connectors to create the ability to connect amps for four speakers.
How many times can you split the signal before it starts to degrade?
.
Your preamp has pre-outs for amps for two speakers, you use two "Y" connectors to create the ability to connect amps for four speakers.
How many times can you split the signal before it starts to degrade?
.