One of the things I wonder about when folks say things like 3 12 volt will be better than 4 6 volt is their ability to read
Group 24 MARINE/deep cycle are 12 volt about 745 amp hour so 3 of them is what 225 amp hour.. and you can only use about 20-25% of that before serious damage starts happening so you have 56 amp hours of usable power.
ONE Pair of GC-2 ONE PAIR is 220 amp hours (Give or take 10) and you can use half of that .. or 110 amp hours. Roughly TWICE the usable power
TWO pair of GC-2 440 amp hours and you can use 220 of it.
Now if you are talking Group 31's.. 130 amp hours so 3 are 390 amp hours and you can use 20-25 percent with MARINE/deep cycle so 67.5 amp hours.
Four GC2 still better
GC-12.. I do not know the amp hour capacity but those are True DEEP CYCLE (GOlf Car) batteries I think around 150 anp hours so 3 of them WOULD be .. about the same as 2 pair of GC-2.. About the same.
NOw if you get 12 volt 4D Deep cycles (220 anp hours) you are talking.
But when you talk about 12 volt batteries WITHOUT SPECIFYING WHICH you are speaking nonsene... As you can see there are many (I only touched at how many) When we speak of 6 Volt this is also true but in this forum we mean GC-2 Golf Car Batteries...
NOW you can get true DEEP CYCLE in AGM 12 volt.. but $$$$$$$$$$$$
li-** types also exist even more $$$$$$$$$$$$$$
But the best bang for your buck is the good old GC-2 nothing beats it at this time.