If you add a second 1/2/both it would allow you to start the engine
from it so that you might on a rare occasion use the whole works to crank the engine
. It may seem unlikely but I needed it just one time. I would not have started the engine otherwise. It was the last day of a 18 day trip and the last time I ever started the engine using any of the batteries. 6 golf cart batteries died on the very last day of the trip. I bought the battery monitor
shortly there after.
I would also agree with AnotherT34C. You want a battery bank to be of similar parentage and date of birth. The the problem is one cell with a lower than normal voltage level will drag down the entire bank. It's not much of a drip but it signals the regulator
to send a little more power. This has the effect of overcharging the rest of the cells in the bank causing them to self destruct sooner until all cells reach an equal level of destruction. It happens faster than you think.
It was (and is) a common problem with flood cell batteries. The idea is to use the 1/2/both switch to "switch out" on or more batteries that you notice going bad (having to add way more water
to one cell vs another). Culling the herd to save the flock. Switches are easier than stretching the battery cables
that don't reach. At the time it happens it won't be a good time.
If you have three unequal batteries of dissimilar parentage or birthday then I suppose go with the two closest in age and make them sisters and use the remaining to start the engine. If all three batteries are way different then it's not going to matter and you'll just wear them out unequally (meaning early). I just would never replace two without replacing them all or at least making the best of the three the new starting battery and making the two new ones the new bank. If right now the house battery is at all old I would go for two new ones and start your trip on high ground (battery speaking).
2 - 1/2/both switches could drive you nuts. It is something you really really don't want to screw up. You need to draw a picture as well as measure the wires. The wires should all be nice and neat with heat shrink tubing and all pro crimped to as short a length as possible. It takes a while to do this properly.