I dont understand that though: If the 3 power units were connected to a common DC buss, you would think they would - or could at least easily be made to - have equal power processing thus be a balanced system. Now if the inverter had 3 MPPTs, than it is very understandable that each one would no process the same amount of power, however I have never seen an inverter with 3 mppt's.
Well, it's sort of a chicken and egg situation. To make sure that the three inverter sections with the same DC supply produce the same ouput power is not terribly hard, although there would be a margin of error. Differences in component tolerances would require an actual comparison of output rather than just an intention to make the three units match.
But with a grid interactive inverter if the three grid wye voltages are not perfectly matched then matching power output would not result in matching currents.
And if you just match current, the easiest way to do that would be to sense and minimize current in the neutral. But if there is no neutral, you cannot do that.
Finally, if each inverter section is trying to match the grid's line to neutral voltage for that phase you surely need to have access to the neutral for voltage measurement even if you are not intending to run current through it.
If the three inverter sections each go line to line then the grid reference can be taken from the three lines and no neutral is needed.
Among other things the manufacturer will have to take into account during the design process that line to line inverter sections will have to operate at a higher voltage than line to neutral inverter sections, requiring a higher DC input voltage, which will either put a minimum on string voltage or require an MPPT circuit that can either boost or buck the DC voltage.