Replacing a battery in a set

Wow! Where to start...
Hoppecke only makes about 30 or so different batteries.
So without knowing any specifics I will answer in a generalized way.
"Testing the acid with a hydrometer" - "3 barely register anything."
Take a little of the electrolyte, about 1/4 teaspoon, from each of the three batteries and make three puddles, one from each battery on some wax paper, then sprinkle a little baking soda on them.
Do they all three "fizzle" rapidly?
If yes then the electrolyte may still be good and may not need replacing.
Have they ever had their plates exposed to air?
Have they been regularly equalized (once a month)?
Are all connection series/parallel clean and tight?
Was the "bank" sized right so that the percentage of charge never goes below 50%?
Do the stay at a low percentile state of charge for very long (2 or 3 days) without fully recharging?
Have you tried recharging just those 3?

What does the manufacturer suggest?