The 3rd one is the drain for the water lines inside the MH. Way to check is turn off city water and open the plugs (no water in fresh water tank) then turn on city water and you will have a good flow then mark that one. Just helps in referencing in future.
One is your water tank drain.
One is the water tank overflow
And not sure what the 3rd one is for.
Surprised to hear that. It is the low point drain. Some units have both hot and cold water low point drains. Only one is needed on your coach for both hot and cold because the winterization valves, when set to winterize the coach, tie the hot & cold together (one system = one drain).
This drain is under the bed,there is a valve under there that needs to be turned..it is very slow..To drain your water lines just go to the outside shower and open that up.be sure to pop the water heater relief valve to let that water out...I had one of these for many years,,..You said you are a new owner of this machine..It may be the one I traded in . Did u get it in Texas down around Dallas?