[JohnFen]

They're called "three-way" because there are three elements to the circuits they're used in: two switches and a load.

...or so I was taught, but that explanation always seemed a little dubious to me.

[cyberge99]

I thought three-way was for three possible states: up/down, down/up, or down/down(up/up). Switch 1 up, switch 2 down, etc.