And on tractors or heavy equipment with very little electrical draw, we usually tie the third wire to the battery wire, like in all the pictures and diagrams shown. This effectively makes it a two wire alternator, which we get away with as long as the main cable is in good shape.
The voltage regulator for one of these delco's is about $5-10 and can be swapped on the machine in very little time. That regulator is the only difference between one, two or three wire, and even some "ONE WIRE" regulators will have the two terminals, even though it will work without them hooked up. confusion never ends.