The Powerwall2 can be installed on a single phase of your 3 phase supply as long as you don't want 'whole house' backup.
There are 3 things you need to ensure:
1. The Tesla Gateway monitoring system has a Current Transformer (CT) on all 3 consumption phases.
2. The Powerwall's Battery management system software is configured to discharge enough power on the battery phase to make up for the other 2 phases' consumption. i.e. it adds together all the phases.
(Note: your installer needs to ensure your DNSP allows this mode of operation).
Then your 3 phase meter simply nets out the consumption.
You get all the self consumption benefits as if the battery were on all 3 phases - as explained here:
3. Any loads you want to be backed up need to be on the same phase as the Powerwall.