Light switch question

There is room with a light bulb in it, there are no windows in the room. There is one door, the door has no window in it.
Outside the room there are three light switch's, you cant see any wires or take them apart.
Only one switch turns the light bulb on, how can you tell which one it is everytime.
Oh and you can only open the door once and it closes automatically. The switch's are round a corner so you cant hold the door open and turn the switch's.

This isn't to hard(can't be i figured it out).