I have three light switches (side by side) in the livingroom of a house I just bought. One switch powers the outside light, the second powers the light on the ceiling but I don't know what the third one is for. I tested the wires running to it with a voltage tester and they work. I tested all the wall outlets in the house to see if it controlled one of them but to no avail. If anyone knows how I can find out where this runs, it would be greatly appreciated.

It sounds like an entranceway from the exterior. The code required a switch to control an outlet for a floor or table lamp. Hallway or stairwell light. It may also control an outlet somewhere on the exterior of the house. Since you know that everyting works, shut off the breaker to those switches and see what does not work. By process of elimination you should be able to trace it. I would also remove one of the wires from the switch. There is a remote possibility that the switch is bad and will not shut off the power. Any other test requires equipment that you will never use again, so, live with it in hopes that one day you may stumble upon it and feel like a real idiot (I know the feeling from experience).

Have fun......

Don't take this the wrong way but did you test both the top and bottom of each duplex? The switch usually only controls half of the outlet. The other half is always hot.

Was there ever a fan/light where the light is now? That third switch may have been a separate control just for the fan.

Found it! The two switches to the left were actually on a different circuit than the mystery one on the right. I found the fuse that controls the mystery switch using a voltage tester and removed it. Through a process of elimination I found that it controlled the outside plug. This had never crossed my mind since the outside plug was at the other end of the house.
Thanks