There are, or at least should be, two valves, one for black tank and one for gray tank. One thing I can think of is maybe sombody added a 3rd valve on to the hose hook up connection. This 3rd valve is sometimes added when one of the other valves leaks. So look to each side and behind of the single valve to see if there are not 2 more valve handles or at least a box and place where the valve handles were. I think it likely someone had valve trouble and "fixed" it with a 3rd valve. I would surprised if it left the factory as a one valve model. Both the original gray and black water valves are the same size and reside well under the trailer in an enclosed area with a handle through a little box on each side of the hose connector. So if you can actually see a valve it is a add on.