3 three channel system, anyone?

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.

Magfan...There were two basic approaches to early multichannel...matrixing with many variations but most notably SQ, and the discrete system you mention. Information for the rear channels was carried by FM modulation of a carrier between 20 and 40 KC. Because it was FM there was no requirement for low distortion in the 20 - 40 KHz range, so long as the carrier frequency could be tracked. The need for response to 40KHz spured great improvements in phono cartridges which benefit us today although the multichannel scheme was a bust.