Logic riddle:

In a house, there are three switches on the ground floor and a light bulb in the cellar. I cannot see the light from the ground floor.

What is the minimum number of trips to the cellar I need to make to identify which switch corresponds to the light bulb?

We assume that the bulb in the cellar works, that there is power to the house, and that one of these three switches will switch on the cellar light.