Not sure what your three valves could be, but on my C three valves like that drain (1) the fresh water tank (2) the water lines and (3) the hot water tank when it is not bypassed. Draining the water lines really requires opening all the hot and cold water faucets so that air can enter the lines at that point to keep from forming a vacuum holding the water in the lines and preventing drainage.
I suspect your three may be the same.
It would be helpful to get a correct answer to know what kind of rig you have. Make? Model?