It means you should see a doctor.Increased size can be caused by different reasons.Mumps, Cancer etc. I'm guessing the feeling of a third testicle is actually an inflamed epididymis. However I'm no doctor^^
It means you are the E.T. (Extra Testicle) The only procedure that can cure this is lancing. The 3rd (usually abnormally large) testicle must be punctured and drained. Be careful... If you hit the wrong one, you'll immediately think it was a fatal error.
It probably means someone miscounted and somewhere some poor guy only has one.
You're one lucky man.
Gosh...that's a genetic deformity. You make consult a surgeon to correct such abnormality.