the white wire is (probably) common/neutral and needed to make the indicator work when the circuit is closed...
the other wires are the 12V power line (in) and load (out) to the heater thing.
if two terminals are X and one is Y... I'd assume the one (Y) is the different one.
But I don't assume anything with electricity and suggest you don't either.
almost any simple VOM can test the continuity of the switch. it will also verify the line & load wires. use one.
Bryan. 2000 Georgie Boy Pursuit.