Single pole and three pole switches look the same when installed but are used for different purposes. Closer inspection reveals that while a single pole switch has two terminals, a three pole switch has three.
A single pole switch controls one hot, or constantly live, electrical wire. It has two positions: on and off. In typical home wiring, one terminal of the switch is connected to the hot supply wire from the electrical panel, while the second terminal is connected to the hot wire of a light fixture. Both wires are usually black.
More commonly called a three-way switch, a three pole switch controls three wires. Although it only has two physical positions, one position changes from on to off, depending on the position of the other switch in the same circuit. A three-way switch can be wired several different ways using different colored wires, depending on the switch locations relative to the lights they control.
Single pole switches are most commonly used in home lighting circuits to control one or more lights or fixtures from a single location, such as the entrance to a room. Three pole or three-way switches are used to control one or more lights or fixtures from multiple locations, such as the top and bottom of a flight of stairs.