A three-terminal on-off-on switch should have a common hot (often in
the center, but not always), and two “load” terminals (let’s call them
#1 and #2). The incoming power should go to the common hot, and #1
should go to the power side of the first circuit. #2 should go to the
power side of the other circuit. Both circuits should be properly
grounded, and consideration should be made for any resistance units
needed. (LEDs require considerably less amperage than incandescent
bulbs, and often require a step-down transformer of some sort.)
Thomas Dayton
Assistant Manager
J S Auto Supply in Jamestown, N.Y.