3 battery setup with on-board charger question

[Advance]

I have this setup... Minn Kota Precision On-Board Charger https://www.amazon.com/dp/B01ACPM8O8/ref=cm_sw_r_cp_api_glt_fabc_130X4RCYH5Y7ECZRBVWH?_encoding=UTF8&psc=1

It’s been great for the one start battery and one house battery. I now have one start battery and two house batteries because of upgraded stereo/amps. The on-board charger has 3 banks, but my installer used only two banks. One for starter battery and one for both house batteries in parallel. Can’t I use the 3rd bank for the additional house battery? That way each battery is hooked up to the charger. The installer wasn’t familiar with on-board charger and wasn’t sure.

 

[HangOutdoors]

Well the way I have mine wired, I have 4 batteries and a Minn Kota 4 bank charger and each battery has a bank.

 

[Advance]

In the picture I posted are two batteries closest. The smaller one is the start battery. There is a yellow wire connecting the negative on the start battery to one of the house batteries. Does that make them parallel? I had someone install these and want the house and start batteries to remain independent.