I believe one of those terminals will be "ignition" and the other will be "accessory". The ACC terminal will power things such as the instrument panel, heater motor, radio, or anything else that needs to be switched on and off without being directly connected to the ignition system. The third terminal connects to the back of the ammeter to receive battery voltage. A simple ohmmeter or even an ice pick test light should help you decide which terminal is for incoming power.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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