Just my guess based on what I know from other boats. One battery works each engine and one for the generator. The two engine batteries will work the 12v interior stuff — lights, radio, etc. They will be charged while running the engines.

You should have a battery charger on the boat. That will keep all three charged when plugged into shore power and the charger is on.

Depend on your location and use, you may want to use the cheaper regular lead batteries — about $150 Or less.

 

I had a 2000 290, which is essentially the same as your boat. If the wiring is still same as stock, the single battery serves the starboard engine, and the pair serves the port engine, generator start, and house loads. The pair needs to match. I usually ran with AGM batteries, starter battery for the starboard and deep cycle for the port/gen/house pair. All three batteries should be the same chemistry (lead or AGM, not a mix).

Shore power charges the batteries when plugged into power, and the alternators charge the batteries when under way.

As for life span, it depends...
Charger: Do you still have the original Guest Charger/Converter? As it ages, it can kill batteries. I replaced the Guest with a Pronautic 1240P model that gave me longer battery life.
If the batteries were standard lead cell, they need periodic maintenance (the batteries need to be topped off). If not topped off, then the batteries cannot hold a charge. If left that way for too long, the damage cannot be un-done.