As long as the speakers are 4 ohms each and the amp has 2 channels and is 1 ohm stable, this works. Three 4 ohm speakers in parallel is 1.33 ohms nominal. A single channel amp would need to be 0.5 ohm stable to run six 4 ohm speakers.
If two speakers are 4 ohms and one is 2 ohms then you have 1 ohm and the amplifier requirement is the same in stereo or mono.
I agree that mixing speaker types is never a good idea, but that isn't the question.
And I didn't read the entire thread, just the first page. I apologize if I am restating what has already been said.