To have a 3-phase system all MultiPlus have to be connected together and configured for 3-phase.
In this setup they should be connected to the same battery.
one battery in a 3 phase system means: put your three batteries in parallel to the multiplus system via a busbar (i.e. lynx power in)
In that configuration the battery with the lowest capacity does not limit the system. The three batteries act as “one”.
Have fun
Marc
Andy from the off-grid garage (youtube) shows this in his setup.