Hold on… This is a lot to take in. And reply isn’t short either 
First of all: I agree that “punish the grid” is usually the easiest solution viewed from our side of the wire - it’s not really up to us to balance the grid. Well, ish.
I expect grid operators in the future to charge for (excessive) phase inbalances, like pushing back 4kW on L1 and L2 to compensate for an 8kW load on L3.
In the past they started giving households single phase 40A connections here, exactly for that reason.
Because then they can spread the load by connecting House1 to L1, House2 to L2, House3 to L3 andsoforth, vs the houseowners all hooking up their jacuzzi’s to L1 and the poor bloke at the end of the street his TV starts to flicker due to low line voltage 
Secondly, the scenario I described can pretty much occur only during a bright sunny day, not when you’re inverting from batteries to power your house or are simply running from grid.
Batteries full, no load, PV inverters pushing all they can to the grid.
And if the neighbors (or you) all connected their single phase inverters to L1, that one’s going to sweat.
The Multiphase regulation page states that in “Total of all phases” mode, excess power (from PV) will be compensated for by charging each phase equally to a total of 0 on the meter.
But when batteries are fully charged, the excess is sent out as is.
After this, the page gets a bit fuzzier.
It states that, in “Individual phase” mode, each phase will be balanced to 0 but doesn’t explain further.
What I’m assuming is that the same will happen as in a single phase system: with 7.5kW PV produced as 3x 2.5kW per phase, 3kW load on L1 & L2, 1.5kW load on L3 - Multi3 will (re-)rectify 1kW, transport it over DC to Multi 1 & 2 who will each invert 0.5kW, so each phase load is at 0.
Or in a more extreme scenario: 3kW injection on L1 and 3kW load on L2 will cause Multi1 to rectify 3kW for Multi2 to invert again.
If I understood correctly, in case of pure excess of PV power, no phase balancing will be done.
So injecting 3kW on L1 with no load on L2 or L3 will simply be injected as such.
Multi1 will not rectify 2kW for Multis 2 and 3 to invert and push back to the grid as 1kW per phase.
For the record: I totally agree that doing so would be very ineffective and a pure waste in conversions and needless wear on the Multis.
But what if, for some reason, this would exactly be what we want to do ?
Possible reasons:
- somebody really, really, REALLY likes to see a balanced net
- due to excessive inverters on L1 in the street, voltage on L1 is at 250V, PV inverters are shutting down and we get no injection fee.
If L2 and L3 are still at normal voltage levels, we could rectify 2kW of the 3kW on L1 and invert it out equally as 1kW per phase.
Or rectify the full 3kW (because L1 is not accepting any power anymore) and inject it as 1.5kW on L2 and L3.
Again: I agree that this would be A Bad Idea, but I would see some people do this.
Early PV installs here were granted “Green Energy” certificates worth up to €450/1000kWh (produced, not injected) for a period of up to 20 years (panels installed between 2006 and 2009, after that the value decreased and the system is now terminated for new home installs).
So somebody with a 10kW PV system from 2009 could earn ~€3800/year for 20 years.
But when their inverter shuts down due to too high voltage on the grid: no production, no certificates, no money.
As for myself: my install is way too recent to get any certificate or support or bonus so my interest would mostly be out of curiosity (in grid balancing).
For PV power produced by panels connected to an MPPT, Victron could check if they can shift the inverting part more to a Multi with lower line voltage.
For PV power produced by an AC PV inverter, maybe vendors like Fronius or SMA could see if they can shift the produced power more to a phase with lower voltage (if they don’t already do).
Using a Multi to rectify power from an AC inverter to re-invert it on another phase would be suboptimal and kind of silly. An easier solution there would be to re-arrange loads and/or PV inverter on other phases.
In your example you’re basically using the grid to balance the inverting power of your Multis.
That works of course, until the grid operator starts putting penalties on that
Like people here with old analog grid meters who end up with zero net usage per billing year because in summer they use the grid as a battery for consumption during the winter: they are paying a fee for that now, since 2015.
On a totally unrelated note: your flow diagram is absolutely gorgeous - how did you make that ?
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