Are you going to add the third battery or remove one of the existing and add that one in there? If your going to add it, you will be fine but do not remove one of the factory batteries and put that one in it's place. The diesels do not like that and it will kill both batteries more quickly. You can fit both stock batteries on the passenger side and add the third battery to the drivers side but it takes a little fabricating. You need a certain CCA (cold cranking amp) for the motor to turn over quickly in cool to cold weather. I doubt that 600 has the correct CCA rating. The factory batteries are 750 CCA each.
No, it does not matter which battery you connect to but I always connect to the passenger side because it has the charge cable from the alternator hooked to it and then a power wire to the drivers side battery.
The vehicle has two batteries already, why do you need a third or want to just change one out?