My circuit breakers are located on the run/charge panel located in the battery compartment. I have a 3 bank CSI built in charger, so I never use the run/charge switch. The switch simply places the batteries in series when in the run position and places the batteries in parallel if you were to charge your batteries by plugging a charger in the run/charge panel, which it appears you do not. So all you would have to do is simply add a 3rd battery and wire it in series. It would be done most simply by wiring the 3rd battery in between the 2 existing batteries. You would do this by placing a jumper from battery 1 neg to battery 2 (new added battery) neg and another jumper from battery 2 neg to battery 3 (old battery 2) pos.

I got my upgraded circuit breakers from BH Electronics (original supplier) located in Atoka, Tenn. Sorry I don't have their number, you will have to look it up on their website.

When I replaced the circuit breakers, since I do not use the run/charge feature, I removed the circuit to make things more simple. When you remove the run/charge panel, you can figure out how the switch/wiring works