Tesla has so much going on that I don't see native 3 phase gateways to be a big money maker, so lower priority. Tesla has so many irons in the fire, imagine how small of a slice of the US population has a 3 phase service. Generator support for instance is much more important imo.
Its a good question whether Tesla will release even a 400A version, let alone a 3 phase, 400A version. Not sure you could tolerate a 2 Gateway topography anyway, its not ideal.
Tesla is famous for going after the largest area under the curve for the investment. I would not be surprised that less than 1% of customers would be served by a 3 phase product.
As to the technical ease, its not exactly just like any 3 phase inverter. A typical home with 3 phase PV inverter will have a blend of both single phase and 3 phase loads. A typical PV inverter sends balanced 3 phase power back to the grid, but a design like this needs some balancing across the legs as well, since you will be operating in off grid mode and cannot have an imbalance.
There are rumors of 3 phase non backed up Powerwall designs being tested, and I imagine this is one of the difficulties they need to address. I'd make friends at Tesla and convince your AHJ to let you do a Powerpack.