I agree with what Hellosailor posted.
You should be able to use the 1/2/both/off switch to turn off all power (except bilge pump which should be wired direct and fused).
It looks like the 2 batteries in your picture are the same. I would parallel them and put them on #1 on the switch and use them as "house" and take the third battery and wire it to #2 as a start battery. That way you would be drawing from a larger bank for both house loads and the inverter. It also simplifies the system which is always a good thing.
This assumes that the other battery is not a larger deep cycle - the one used for the inverter.
Living aboard in Victoria Harbour