Lance, (Colton, CA)
The short answer Lance, is yes, it is possible to add a third water pump switch, but it’s necessary for the new switch to be a 4-way switch (four terminals, instead of three), rather than simply an additional 3-way switch. And please don’t confuse a 3-way switch or a 4-way switch with a switch that simply has three or four terminals! They must be internally constructed specifically as 3-way and 4-way switches. As the RV is equipped now, you have two, 3-way switches in the circuit to power the water pump. When adding a third switch into that existing circuit, you’ll have to obtain a new 4-way switch. Be sure it’s rated to carry the DC load of the water pump.
It will be necessary to electrically “insert” the new 4-way switch in between the two existing 3-way switches. Any number of 4-way switches (literally), can be added to any circuit, as long as a 3-way switch begins and ends that circuit. Theoretically, you could have a hundred switches for the pump. The first and last would have to be 3-way switches and all the rest would be 4-way switches.
Also remember, you will only be switching the “hot” wire to the water pump; the ground wire does not connect to any switch. Just be sure you have a good, frame ground connection for the water pump and the battery. If it’s too restrictive to locate the existing wiring or if you have difficulty in determining how the existing switches are now wired, another option is to simply start afresh with newly run conductors. Take a look at the existing water pump fuse; if it’s a 15-amp fuse, you’ll need to run 14-gauge wires, if it’s a 20-amp fuse, you’ll need to use 12-gauge conductors.
The common, center terminal of a 3-way switch will be colored differently from the other two terminals on the same switch. Some may even be labeled. Typically, the positive feed wire from the fuse panel is connected to this common terminal on the first switch in the circuit. Likewise, the common terminal on the other 3-way switch is what ultimately connects to the water pump.4-way switches do not have a “common” terminal, per se, but instead, has color-matched pairs of terminals. Remember, a 4-way switch is not JUST a switch with four terminals; it’s internally constructed as a specific, 4-way switch.
As seen in the diagram above, the two traveler wires, (those wires that run from switch to switch), from the first 3-way switch connect to one of the color-paired terminals on the 4-way switch. And the traveler wires from the 4-way switch connect to the two, outside terminals on the last 3-way switch. It is important to connect the traveler wires correctly at the 4-way switch. Hopefully, the RV manufacturer used different colored wires when first installing the two, 3-way switches you have now. That would make your task much easier. Oftentimes, however, it may prove easier to just run all new wires. You’ll have to be the judge on that one!