The typical 3-port MIDI adapter (that is 3 MIDI outs on the same 16 channels) is merely a 1 MHz crystal to provide the clock for the Mac's SCC chip to generate 31.25 kilobaud and some DIN jacks. These are limited by MIDI's slow serial rate. There are "32 channel" adapters that use both ports, and can issue simultaneous events if the Mac's MIDI sequencer has sufficiently tight timing. Then there are the adapters which contain their own CPUs and multiple MIDI serial ports such as the Opcode Studio 64 and the MOTU MIDI Express. The latter communicate with the host Mac at a higher baud rate, like 115 or 230 Kbaud, so should have tighter timing possible between channels. In actual practice I don't know what the timing accuracy is, since it depends on routing and processing within the interface box.

In the practice of acoustics, it is generally assumed that sounds are perceived as simultaneous if they are within 25 mS of each other. Much more than 25 mS apart we hear an echo or arpeggio.

In practice, driving 3 synths from Vision running on a 512K Mac with a simple MIDI adapter, I haven't noticed any problems unless I let something like pitch bend data clog the channels. I suspect MIDI performance on a Classic will be fine, though it's hard to predict whether it can hold <2mS jitter. Too many variables, and 2mS is about 10X the accuracy most folks can hear.