If you or someone you know has ever changed a 3-way switch and not gotten the wiring put back together right, you know how frustrating it is trying to figure out how to get it to work again. You may also understand the pain of having to pay an electrician to put the wiring right.
TURN OFF THE POWER AT THE CIRCUIT BREAKER Remove the 3 wires from each of the switches and make sure that none are touching anything.
IN THIS STEP YOU WILL BE TURNING THE POWER BACK ON, DO NOT TOUCH ANY OF THE WIRES PREVIOUSLY REMOVED WITH ANYTHING BUT THE METER PROBES AS INSTRUCTED. Turn the power back on at the circuit breaker. Set the electrical tester to the 120 volt AC (alternating current) setting. Locate the wire which is supplying power to the circuit by touching 1 of the probes to the ground (bare or green wire) and the other probe to each of the 6 wires one at a time until you get a reading of 120 volts on the electrical tester. This is the common or hot wire. screw a wire nut on the end of this wire for identification purposes. TURN THE POWER OFF AT THE CIRCUIT BREAKER
At the switch where you found power in step 2, put the other 2 wires you removed from the switch together and wire nut them. Now go the other switch location. Set the electrical tester to the continuity or ohms setting. Touch the probes to the 3 wires removed from the switch until you get a reading on the tester indicating continuity. These are the "runners" of the circuit. Connect these wires to the switch, 1 wire on each of the brass colored screws. The third wire is the common wire that feeds the light fixture and is connected to the black screw on the switch.
Go back to the first location and attach the single wire which was identified as the hot or common wire in step 2 to the black screw on the switch. Connect the other 2 wires, 1 to each of the brass screws on the switch. Replace both switches and covers.
TURN THE POWER ON AT THE CIRCUIT BREAKER. Check the operation of your light. Congratulations, you can now wire or trouble shoot a 3 way switch circuit.