Riddles: 3 light bulbs (Answer)

 

Light bulbs generate heat when they are turned on.

Therefore, turn on switch 1 and 2. After 5 minutes, turn off switch 2. Enter the room at map the bulbs as follows:

- Switch 1 belongs to the lit light bulb
- Switch 2 belongs to the warm light bulb that is turned off
- Switch 3 belongs to the last light bulb