How a 3-Way Wall Switch Works
Light switches are simple in design. Current flows through a switch to the load, such as a ceiling light. When you flip the switch off, it breaks the circuit and interrupts the flow of electricity. A basic light switch has two terminals and sometimes a ground terminal. The hot wire from the power source is connected to one of the terminals. The hot wire going to the load (such as a light) is connected to the second terminal. A 3-Way switch is different in two ways. First, it has one more wire connected to it, and second, instead of being on or off, it switches which wire it routes the current to.
A three way circuit allows you to operate a fixture or outlet from two different locations. You must use two switches and both switches must be a 3-way switch. A standard switch simply breaks or makes a circuit, it is either "on" or "off". A 3-way switch routes the current down one of two wires called travelers. When both of the two switches make contact through the same traveler wire, a circuit is made. This is how each 3-way switch can, at any time, turn a circuit on or off. Each switch can reroute the current to make or break the circuit.
Do I Need to Replace my Light Switch?
When a light switch fails, symptoms can include a loose or wobbly switch or it may be stiff or difficult to push. Lights that flicker may indicate a switch that is shorting. A switch that has failed completely will fail to turn on or in rare cases fail to turn off a circuit. With a 3-way switch circuit, one switch may fail but the other switch continues to work. However, identifying which switch has broken is not always obvious. If both 3-way switches are the same age, it may be worthwhile to replace them both at the same time.
If you do need to replace a wall switch, it is fairly easy to do yourself. Here is an article describing How To Replace a Light Switch.