The Lightbulb

inside a windowless room is a lightbulb

outside are three on/off switches - 2 are props, one is functional

you may configure the switches anyway you like

you may open the door and go inside the room once, but once you come out, the door must stay shut

do whatever you want outside, do whatever you want inside

how can you figure out which switch is the functional one?

note: the answer is logical and has nothing to do with the wiring, or somehow seeing the light from outside, or getting help from a friend