One wire comes in from the power source, and one wire goes out to the light (s). A 3-way switch adds two terminals for travelers, which connect two 3-way switches together.

There are two types of light switches, says Donnelly: a single-pole light switch, which has two wires, and a three-way light switch with three wires that usually look identical.

In the diagram on the link they are using a three conductor cable after the first three way switch. Black, Red, and White with a separate ground.

So the existing switch has 2 terminals on one side, and 1 on the other. The 2 terminals have a black sheathed (B) and red sheathed (R) wire attached. The single terminal has a grey sheathed wire.

To make the wiring work flawlessly, so your husband becomes a lover of these switches, you install the same special three-wire cable between all the three- and four-way switches in the circuit.

One wire comes in from the power source, and one wire goes out to the light (s). A 3-way switch adds two terminals for travelers, which connect two 3-way switches together.