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Research polyphony on a modular synth. It's a lot more complex to do it right than you'd think. Typically, you'll use multiple VCOs stacked into a single voice. If you're looking at using three single voices in a polyphonic way, you'll need three VCOs, three VCAs, three filters, some sort of system for assigning new notes to an available oscillator. You'll have to tune each set of VCO + filter exactly the same.

I think you'll find a modular solution to be pretty inefficient versus a DCO polysynth that just works out of the box.