Do we know what the other switches control? If the other two switches are known to control lights on the other two floors, and assuming all lights start as off, then I know the answer. Flip switches until you find the one that controls the bottom floor. Since you're there, you know instantly. Then, flip one of the other two and go up to the second floor. If the lights there are on, you know the one you didn't flip is the third floor. If they're off, then you flipped the one for floor 3.