Controlling a single lightbulb from multiple switches
Just to be clear, a 3-way switch (i.e., one light bulb controlled by two switches) is wired up like this:



simulate this circuit &ndash; Schematic created using CircuitLab
For a 4-way switch (three switches controlling the light-bulb) you start with the same two switches as shown above, but you add a DPDT switch in the middle. That DPDT switch is set to route electricity straight through in one position, but cross connect the upper and lower wires in the other position.