You don't need to go back upstairs.
You turn on one light. Let it sit for about ten minuets.
Turn on one of the other lights and go downstairs.
The one you flipped first will be hot, but the one you flipped second won't be yet,, and the third one isn't even on. So 1 switch was connected to the hot bulb, one to the on-but-not-hot bulb, and one to the bulb that wasn't on. As long as you remember which switch you flipped, its simple deduction.