Dx grunt, depends on the length of the cable, the longer the length the thicker cable you will need. If you are going from front to back etc I would go atleast to an 8 B&S cable or even go up to the 6 B&S, run it into a fused distribution block and you could run other things off it as well as the HF, just a thought. You will obviously need to charge this 3rd battery as well and if in the back like you said I would use a minimum of 6 B&S. Are you going to join the batteries up into a triple battery setup or have one of the ARK battery boxes with the charger already in it.
Sorry if I have added to the confusion but it can cost a lot of money to fix a stuffup...I know as I have added more wires and taken wires out changed things around and it has cost me a few dollars.
cheers
Greg