You Have Been Hacked

It might be too late, but you could still save yourself complete this riddle and you MAY be okay.

"You are standing in a hallway with three light 
    switches. One of the switches controls a lamp 
    inside a closed room. You cannot see inside the 
    room. You may enter the room only once, and you 
    must determine which switch belongs to the lamp. 
    How do you solve it?"