> How many residential appliances benefit from three phase, and is the benefit outweighed by the additional cost of more copper for the additional wires?

due to the higher voltage in europe, conductors tend to be quite a bit thinner than in the US, to carry the same amount of energy. That being said, our electrical installations tend to be much beefier than most US residential, so a lot of copper is used. in Denmark for example standard for houses is to be fused at the roadside box for 3x~30A