The basic concept of a triple-gang switch is simply three individual switches stacked on top of one another. The triple-switch is convenient because it only requires a single-gang box instead of a triple-gang box. The switches are mounted one above the other, and the switch activates by moving the button horizontally instead of vertically. A double-gang switch is also available.
The switchbox you will be using will have four black and/or red wires inside. One of the wires is the power feed. It is also called the "hot-wire." The remaining three go to the three light fixtures that the switches will control. They are called "switch-legs." Use the voltage tester to determine the hot wire before turning off the power. It is the only powered wire when the main-breaker is on.
Turn off the main-breaker. Put a piece of tape over it to prevent someone from turning it on.
Strip the three switch-leg wires 1/2 inch from the ends. If the switch has push-in connectors, push the wires into the three slots labelled "load." If it does not have push-in connectors, use the needle-nose pliers to bend the end of the wires into a loop and insert them under the "load" screws. Tighten the screws firmly.
Strip the insulation on the hot-wire 1/2 inch from the end. Look on the "line" side of the switch and see if there are small metal jumpers connecting the three terminals. If there are, push the hot-wire into any of the three "line" slots. If they are not there, cut three 6-inch lengths of wire and strip each end of the wires 1/2 inch.
Use a wire nut to connect the three wires (pigtails) to the hot-wire. Push the open end of the pigtail wires into the three "line" slots. If there are no slots, use the needle-nose pliers as in Step 3, and insert them under the "line" screws. Tighten the screws firmly.
Gently push the wires into the switchbox and secure the switch to the box. Add the cover plate.
Turn on the main breaker and turn on each switch, to verify that they are working properly.
Better quality switches have push-on connectors. They are much easier to work with, and they will last a lot longer than inexpensive switches.
Electricity can cause serious injury or death. If you are not comfortable working with it, hire a licensed electrical contractor.