Hi MarkI have a 3 phase solar inverter and a PW2.
The PW2 charges and discharges over one phase, but it can monitor all three. I have a single phase critical circuits sub-board that it backups during an outage.
When my three phase solar is exporting the powerwall charges at the full rate upto 5 kW, not 1/3 as suggested above. It does this by monitoring all phases and balancing export on phase 2 & 3 by increasing import on phase 1. So the instant net power draw from the grid is zero across all three phases.
During an outage a single phase solar inverter can recharge the battery, a three phase solar inverter cannot recharge the battery during an outage, but that wasn't important to me so I has happy with that limitation.
I'm doing the same - have a current PW2 on a single phase inverter, but now putting in 3 phase and adding two more inverters and another 13kW on the roof. Interested in how they wired your Gateway...