why do you americans call it a 3 way switch? theirs only 2 ways , if its due to switching from 3 locations, wrong again ,your using 2way and intermediate switching
1) Because a normal switch is 2 way -- on or off
2) There are three scenarios relative to the levers: up/up, up/down, and down/down
(Actually one hallway has 3 such switches which should be wired XOR, but aren't. I'll give the C-student contractor a pass on that. But incorrectly wiring just 2 switches is unforgivable.)
3) Just to bother you Brits, which we have been doing for almost 250 years.
I'll wait for 298 more responses before replying again.
If you need
three switches to control one light, then you add a 4-way switch to the two wires between the two 3-way switches.
Just as 3-way switches are SPDT, the 4-way is DPDT hooked up as a "reversing" switch.
I learned this in junior-high-school electricity class, ca. 1962.
One way
not to do two 3-ways is what I found in an older house belonging to my in-laws when they asked me to fix the dining-room light switches.
Someone had put one SPDT/3-way switch on one wall (above an outlet), and the other on another wall (above another outlet).
Each switch's contacts went to Line and Neutral at the outlet below, and the moving contacts connected to each other through the ceiling light on a single wire.
Unfortunately, at least one switch was make-before-break, so it kept blowing fuses.