The advance of a (Victron) home battery is that you could draw more power (kW) from the battery then is possible from the grid, with a 3 kw MP for each phase it’s less, with a 15 kw MP is 3 times more…(for a certain period of time) all what you need is large battery.
Everything beyond the main fuses (and grid metering) is the domain of Victron.
By that you mean the airflow / required spacing surrounding each Multi plus i assume?
Me too, or have the 3D print file 
Why did you go with 3-phase, out of curiosity? I also have three MPs, but intend to go monophase in parallel, in part because even the HVACs are monophasic, and I don’t want affinity by load. (I don’t have any EV chargers or anything very esoteric, mostly standard appliances, light fixtures, and so on.) What benefit do you get?