So you have three batteries. If they are all in parallel during charging then a single volt meter would show the combined voltage. Do you have three separate banks? If so why? Typically most vehicles need two banks (which may have more than one battery).
Three batteries, one for starting, two for house. The house batteries can be independently switched in or out of the charging circuit, switched in or out of powering the fridge, switched in or or out of the starting circuit, switched in or out of the winching and OBA activities, combined or disconnected from the starting battery in just about any configuration you could imagine. I can jump start myself from either or both of the house batteries, without pulling out cables. Can be isolated when the truck is off, or can be combined into one, two or three battery combos when truck is off. When charging, I can charge one or two or all three in parallel, but starting battery is always being charged when truck is running, unless I delete charging to all. Except that if starting battery is removed from the truck, it won't charge, but I can still start and drive the truck on the house batteries. When charging, it's all one bank, but the bank can consist of one or two or three batteries. So, my voltage gauge and my gauge pod show system voltage when charging, but not SOC of the individual batteries. The Nat Luna battery monitors were supposed to give me info on individual batteries but those monitors are nearly useless.