All three cells?
My swag is battery 2 has a higher resistance, but I have no idea.
I was going to say bad, but what makes it bad if it has the same capacity as the rest, I’d assume it doesn’t.
But that means a cap check and I know that’s not likely and understand why.
Maybe try swapping it with a different battery to see if the waterconsumption follows it, that ought to eliminate wiring?
Try swapping caps if they are the quick-link ones. Could be one is not sealing well.
Check all connections for a bad one, even a poorly crimped lug or any hidden corrosion inside. Check for excessive wiggle in the lug. Maybe hit them all with an IR thermometer looking for any hot spots on the wiring at high rates of charge
Maybe #2 is getting warmer because of position? In the corner and not shedding heat as well, an IR thermometer over everything should show you any hot spots.
But my gut says bad cell, sulfation, shorted. Are all the batteries from the same manufacturer and dates? Are any older than the rest? Best to be all from same lot when you buy them.
When I bought my four GC-2 6v batteries for my house bank I dug through the whole pallet at Sam's Club to find 4 matching ones all from the same production lot and manufacturer date. I was amazed that this stack had ended up together from so many different lots and mfg dates.
Apply a decent load (around 40 A) for 15 minutes, remove it for 15 minutes, and measure the voltage across the battery. If it is close to 4.2 Vdc and no where near 6.3 Vdc, it’s likely a shorted cell.
This can be a dangerous situation as that side of the bank will drain the other side, boiling all the time. At some point you can enter a thermal runaway situation that may cause a whole world of hurt.
Even before charging, a battery with a bad cell should show a fairly different voltage across it compared to the others in the bank. A hydrometer test should show a significantly different specific gravity as well. Either result would be a red warning flag.