You can but I would not recommend it. Mainly you are going to need to charge the trolling battery's and you can do that with one connection charging both battery's at the same time. I would not want to add the third onto that same circuit because you could over load the charger and if on cranking battery is not going to need much charge and you could burn it up because charger is sensing the need for high amps at the front. Those 2 up front would equalize as they charge. Hope this makes sense.
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Charlie Weaver USN/ENC 1965-1979