The 3 wire alternator has the main charging wire, an "exciter" wire, and a wire that runs to the dash warning light.....basically. If you were to unwrap the harness tape you'd find that there is a soldered junction where one of the wires from the voltage regulator is simply clipped and soldered to the big wire that bolts to the back of the alternator. The other 2 wires on the alternator plug run to the voltage regulator, then the blue wire runs to the dash warning light.

When you switch to an internally regulated alternator you can eliminate the voltage regulator and wiring plug on the firewall by simply bypassing them. The plug for the alternator can be altered so that one of the wires "senses" at the soldered junction, while the other wire goes to the dash warning light. You or I would have to look at a wiring diagram to see which color wire goes where.

I did this change when I installed a 94 Camaro 140 amp alternator. It bolted right in place of the original.