All three should be closed when you're using the water system.
The two which are close together are for draining the hot and cold lines in the front of the coach. The one which is located under them is to drain the water tank itself. It's connected to the same line that the pump is drawing from, so when it's open, the pump will not draw water.
You might want to check for open drain valves in the rear of the coach and close them before you try pressurizing the system. Check to make sure the drain plug is in the water heater. If there is a filter installed under the sink, make sure it's either off, or bypassed, or has a filter installed. That will save you from some potential water fountains!
Dallas Peak, MD 'That 70's Guy!'
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