Fwiw, I added a third battery to my rig this week.
It didn't cost anything, and I already had a spare charging bank open, so it wasn't a big expense.

I put it into an above deck side cabinet that used to hold my oil tanks. It is rigged on the seperate charging bank, and I have a standard on/off switch between the battery and one of my primary bus-bars. The idea is that it will be in the off position by default, and switched into the main load only if it is needed.

I'm also going to rewire one of my VHF radios to be run by this battery as a house circuit (maybe I'll switch that connection also on an a/b switch). It's a group 27 AGM battery (as are the other 2), so it shouldn't ever get run down in normal use by a single VHF, but it will give me those last few moments to get a message off if my bilge fills with water off to the point that the other batteries short out.

Of course, it adds another 80 pounds of dead weight to the boat. But it is reassuring to have a backup battery if for nothing else than to start the engines after a long drift.