I assume you are talking about the rocker switch under the tach that chooses L or R? that should have 3 wires as you state. I recently fixed all my door mirror wiring while i had my door panels off, so here are the basics on how the system works. the door switch has power and ground, and depending on position alternately sends +12 or grounds the two wires going to the motor and if selecting up/down it sends power to the solenoid in the mirror gear box to shift the mechanical to drive the mirror up/down instead of L/R. the solenoid in the mirrors will always work simultaneously regardless of which mirror you are trying to move, however the switch under the dash will interrupt the motor circuit on one mirror and completer the circuit on the other to cause that mirror to actually move. the green wire it the one fed from the switch so should be connected to the switch on the side that has only a single pin and full length copper trace across that side of the switch. The left and right mirror wires then connect to the remaining 2 pins (if you get these backwards just flip the switch over).
Here is a diagram with the changeover switch included.
