I stole this from a Mustang Forum... (link at the bottom) - It's worth going to the link and seeing the whole conversation - They also mention a resistor as a fail safe in the alt bulb burns out...

"The exciter wire tells the alternator that the ignition is on and is the "turn-on" lead for the alternator. Power comes to that terminal from the battery, through the ignition switch. Once the alt starts charging, current reverses.

So, until the alt starts charging, battery voltage illuminates the bulb because there is no juice coming from the alternator. Once it starts charging, it generates an opposing voltage so that the net voltage across the light is very low (too low to illuminate it). The bulb is getting pushed about the same amount from both sides for a net near zero."


http://forums.vintage-mustang.com/mo...onversion.html

Im not sure how to help you with the three wire connections - are you sure your current alt has an internal regulator? Maybe you still need to use your old external regulator and pigtail harness then connect the new EZ to that somehow?