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.)
J S Auto Supply in Jamestown, N.Y.