3 three channel system, anyone?

What do you think the center channel is but a combination of the left and right? This is how it was done in the days when 3 channel was common.

Stanwal...If you use a simple Y interconnect you have shorted the Left and Right together. ALL channels will be Left plus Right... in other words Mono. To mix Left and Right so as to get a center signal without screwing up the Left and Right you must use a resistor network.

In my opinion the simplest and best way to drive three front speakers is to use a stereo power amp with the signal of one channel inverted. Bridge the amp with your center channel, and reverse the speaker wires on the channel that you inverted. Your amp will see a low impedance load, but I never had a problem even with 4 ohm speakers.

Another way is to connect the three speakers in a Y configuration. This will not lead to a low impedance load,

Many years ago I experimented with non-standard connections for speakers.
Using 2 amps I had a standard pair of L/R speakers in front using the first amp, my brothers SAE. No problem.
Next, I connected a pair of speakers IN SERIES between the 2 hots of another amplifier. These speakers I stuck in the BACK of the room, maybe 10 feet apart. This amp was my Kenwood from my first 'serious' effort at better sound. The effect was startling. Any music with phase information 'intact' sounded really live. As a matter of fact, albums recorded live were scary. The music would come from the front pair, with little or no sound from the back. As soon as the crowd began clapping, you were in the MIDDLE.
Current over-engineered music lacks many phase cues which renders this probably a moot technique and makes my absolute phase switching on the amp so much extra circuitry.
OH, the technique to which I refer? I got the idea from SQ / QS 'matrix' sound at the dawn of '4 channel'. I think Dynaco (?) sold an adaptor which did the same thing, from a single amplifier.

Magfan...What you did was basically the "Dynaco" setup, also attributed to Hafler. The Dynaco setup also included a Y-connected center front, which increased the load impedance of the front speakers so as to compensate for the series-connected rears that were bridged across the amp. Dynaco also suggested a "blend" resistor at line level to restore Left/Right separation which is reduced by the Y connection, but I always prefered the sound without the resistor.

With certain recordings the results were superb.

I remember ALL the early 4ch stuff being just phase extraction from the original 2ch material. On recordings with mixed or screwed up phase information it worked only ok if at all. I remember LIVE recordings with few mics sounding best since the phase is generally preserved.
For the DIY crowd, BE CAREFUL if you try any of this stuff. If your AMP doesn't have common ground you could fry your outputs and upstream.
There WAS 1 discreet 4ch system, from specially encoded vinyl which needed a special cartridge with freq response to 40khz plus. I heard a demo with 4 people speaking 4 languages, 1 from each speaker. Trouble came in when the HF encoding started to etch away. Someone like me, who never mastered the art of keeping vinyl perfect it was a hopeless proposition. CD's, for better or worse came along just in time.