Three Switches and One Bulb

The problem
3 switches outside · 1 bulb inside · enter room once123sealed roomwhich switch controls the bulb?

Outside a sealed room, there are three light switches. Exactly one of them controls a single light bulb inside the room. The door is closed and you cannot see the bulb from outside.

You may flip switches freely. But once you open the door to check the bulb, you cannot flip any switches again.

How do you determine which switch controls the bulb, with a single entry?