These are used to control three-phase loads such as motors. With a single external handle, three current-carrying conductors can be opened or closed at once. The switches often come with housing for the mounting of fuses to provide overcurrent protection. The switch must have an equivalent horsepower rating to any motor it is controlling the current to.
The three ungrounded conductors are called Line 1, Line 2 and Line 3, while the load side terminals are referred to as Terminal 1, Terminal 2 and Terminal 3.