Triple battery set up question.

[bushnbash]

If you are talking about with the engine running IE there is an Alternator supply then it won't matter a rats which battery as your DC DC charger will 'see' the Alternator.

If you are talking about engine stopped and you have no battery management or isolator it won't matter a rats which battery as your third battery DC DC charger will drain them both somewhat equally

If you are talking about engine stopped and you have a battery manager or isolator that disconnects the cranker if the aux goes low then connect it to the Aux not the Cranker as the DC DC charger on your third battery will drain whichever one it is connected too.

No matter what you do the 3rd Battery can only ever charge at the max of the Projecta device IE 7 amps or 10 amps or whatever model you have so the state of charge of the other batteries won't have any noticeable affect when the Alternator is running UNLESS you have a large capacity unit (say above 25 amps)

Sorry about my lack of knowledge and probably stupid analysis on how I think it would work but hear goes.............
Yes the 3rd battery has a PROJECTA IDC25 DC/DC 25amp wired into the battery box.
There is also a IBS Battery Monitor System and also a big on/off dial under the bonnet, which I was told by the previous owner was to be used when using the winch.
I was thinking that if the third battery was connected to the starter battery it would get a better charge then being connected to the second battery.
Especially if i had been camping for a few days and the fridge was running off the 2nd battery and it was at say 50%(12.2v) . If the 3rd battery was connected to the 2nd battery wouldn't it have to wait until the second battery got charged enough before it would start drawing power from the 2nd battery? or would the 3rd battery draw all the power and the 2nd battery would not charge to it cappacity until the 3rd battery was fully charged. Thanks for any imput as I can quite get my head around it.