Uh, take one bulb out of socket. Turn one switch on, wait 5 mins, turn it off. Turn another switch on and leave it on.
Go upstairs.
1) If no bulbs are lit, the switch you left on is for the empty socket. Feel the other two bulbs -- one will be warm (the light that was on for 5 mins) so the first switch you turned on goes to that one. The cold bulb goes with the remaining switch.
2) If one bulb is lit when you go up, that is the switch you left on. Feel the other two bulbs as per solution #1.
Newly