I'm guessing the calculation would be based the other way, i.e. whether there is an installed base or not. If you have a 3-phase source, not using all the phase

I'm guessing the calculation would be based the other way, i.e. whether there is an installed base or not. If you have a 3-phase source, not using all the phases would limit total power delivery needlessly. So most European residences will want 3-phase charging.