But the only differing cost in that scenarios is the 2 extra conductors to the local transformer, and more likely just to the edge of the property. The 3-phase power is still present in the area, as ideally alternating houses are on alternating phases to balance the load over the phases. The difference in cost would be a one-time hit during installation, and the ongoing maintenance would be the same as 3 separate houses (1 house per phase). That maintenance could be rolled into the cost per watt, so the more you have available the more you can use and the more you could pay.

The installation cost should vary based on what the house wants access to, and the ongoing cost should be the same as every household. A standing charge for the cost of the infrastructure existing is ridiculous when that same infrastructure is what the power company relies on to deliver their chargeable commodity. It's effectively double dipping - how is it any different from ISPs charging for access and then charging for data on top?