8-outs on each synth is a lot! 24, baby!
The M320 is 3-band EQ, with 2-bands sweepable and 1-band fixed EQ. The M520 is 3-band sweepable EQ.
The 3-band sweepable EQ is better, and the 500 Series boards are more high tech, better board than the 300 Series.
If you have 3 synth devices, with 8 outputs each, and you want to run EVERYTHING simultaneously, then you'll need a 24 channel board. If you may not run everything simultaneously, you'd be well set with a 20 channel board.
The dual M30's might not be a practical solution for you, being 8 input mixers. 16 channels might, or might not, be enough inputs.
The recorder or destination device is important. You have to consider that, in the mixer decision, because that would influence the number of busses, or subgroups, you'd want.
I would not want any novice to run out and buy a huge mixer, before he'd considered all his mixing needs, very closely.
Consider the recorder. Is is a 'puter? Is it analog? How many tracks is it? How many tracks will it record, simultaneously?
Are you just intending to do DJ parties with a monitor feed into a PA? Then, I'd say get a Mackie 2-buss stereo mixer. If you want to record, I'd say get the Tascam recording mixers, as I've mentioned above.
Ever consider the 388? It's an 8-track in-deck reel/reel tape drive, and onboard 8 input, 8-buss mixer. Basically, an analog, reel, 8-track Portastudio format recorder. Now, the 388 gives you full 3-Band Sweepable EQ, and 8-buss, where the 300-Series mixers don't. The 388's mixer section would be more sophisticated, w/EQ and buss count equivalent to the 500-Series mixers.
MIX? MIX WHAT? MIX FOR LIVE WORK? MIX FOR RECORDING? WHAT RECORDER? RUNNING HOW MANY INPUTS, SIMULTANEOUSLY? WHAT'S YOUR BUDGET? [LIKE MY CAPS?

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Those are unknown factors, to me, and something I' cant decide for you. Otherwise, I could keep recommending and reviewing mixers, endlessly.
There are many other Tascam mixers, but when you say "vintage", I assume you're talking about the M30, M35, 300-Series & 500-Series mixers. These are the VU-meter vintage 80's mixers. If you consider some LED meter mixers vintage, then there's a whole number of 90's Tascam mixers I've not even mentioned.