Hey Chuck, I got a reply, he's always punctual with his responses. This is what he said:
You need even numbers of speakers for the best match to the amp.
3 x 8 ohm speakers in parallel = 2.7 ohms (too low); possibly will overheat the amp.
In series, it's 24 Ohms (too high); it will work, but won't sound very loud.
If you can add one more 8ohm speaker you can then at least get 8 ohms by seriesing 2 of them and then paralleling the two resulting 16 ohm loads back to 8 ohms.
3 speakers really doesn't work well on these amps.
Now he didn't address a 3 speaker parallel/ series situation. I would check your overall ohms at the end of the hookup with an ohm meter and make sure the speakers have 5-6 ohms. I'm sure he would mention that if it was possible. Your amp should have 2 speaker out jacks, right? you can hook up one to there and two to the other. Sorry his response wasn't helpful.
Maybe Jerry had 3 speakers at high ohms to tame the volume on his 350 watt MAC and have the amp clip? I would bet there is something going on there other than 3 speakers wired parallel/series. Healy could have some kind of buffered system to change the ohm input or something of the sort. It could be that the Mac can see funky loads. Maybe one of our historical electrical gear heads can chime in on how Jerry had those towers wired. Peace bro, Jon