Battery combiner size

I'm looking to add a couple of batteries to my boat in addition to my starter battery and am planning on going the combiner/perko route. The yandina C100 looks like a good deal, but I'm not 100% sure it'll work with a 3 battery setup? Would I need to go for the C150 for two banks, one bank with two optimas, a single battery on the other?

I have an upgraded 100amp alternator, so I figure that should be ok.

Also, my current started (stock) battery is behind the rear seat, but I would like to locate the two new optimas in the very nose for weight distribution reasons. What gauge wire should I run to connect the batteries to the perko and also to each other?

Thanks in advance for the help!

One thing to keep in mind is that the C100 is ignition protected, the C150 and larger units are not.