2nd battery

just looking for a bit of advice, I've got 2 optima batteries and a split charge system. Lurking in my barn is a very good condition leisure battery, could i fit a 3rd battery, i've space, would it charge with the yellow optima when the engine is running? would i be better isolating the 2 batteries so i can charge each seperately, thought maybe a kill switch on the live feed, when connected would charge, disconnected charges other battery etc. what could i gain from 3 batteries as apposed to 2? i will be running a fridge/freezer, laptop, lighting etc, your thoughts please

The more batteries you've got the longer your fridge etc will run for. You mite want to fit a 100amp alternator, though

I've just had an 80 amp fitted at Foleys, would it be a good idea to seperate them for charging?