I just finished adding the 3rd battery switch so I could isolate my House battery.
I actually also installed the 4th battery and tied 2 of the batteries together to make one big one for the house as I could not fit a 4D in there.
Pic shows the battery switches. Works great so far!
I had to make room for it as the area is very tight, but was able to move original battery switches over and the blue seas fit due to it's design.