A 3-way switch is used to turn a light fixture "OFF" and "ON" from two locations.

From a safety stand point. it is highly advantageous for stairway and hallway lighting as it allows the occupant of the home to turn the light ON from the bottom or top on the stairs or from either end of a hallway.

For a complete selection of switches visit:  P&S Switches

The wiring of a 3-way switch can be confusing.  However, there are only 3 possible combinations of wiring that can utilize a 3-way switch and those 3 situations are relative to where the fixture is in relationship to the 3 way switches and the incoming power to the circuit.

It should be noted that the circuit can control more than one light fixture.  Additional light fixtures are run in parallel to the first fixture.

If your application requires multiple light fixtures controlled by the same 3-way switches the additional light fixtures should be wired in parallel to the first fixture.  The circuit should be connected to the point where the first light fixtures wires are connected to the power - after the switches.