I think the multiplus currently try to minimize the own inverting effort.
So, the multiphase regulation will always allow grid-pull on the phase where you have the highest consumption, and use the other two to regulate, so each multi has about the same “dc-power” / inverting effort.
That way, you could (with 3 x 5k multis) theoretically use upto 12kW on a single phase, without having to “grid pull” in a net-metering sence.
My Flow-Diagram outlines that pretty neat:
Adding 1 kW on Phase 3 now only causes about +330 Watts (DC) per multi:
If that multi would now be feeding in, cause Voltage of L3 would be lowest, he would almost already hit it’s inverting limit… Don’t think that is desired. If in doubt, punish the grid 
The system then would have to switch to feedin on another unit, and will be heavily limited in reaction speed due to maximum allowed ramp power.