3-phase domestic supplies are pretty common in Continental Europe, although not here in the U.K.
Germany is such a country: Standard service per apartment / small house is 3 x 63A @ 400/230, i.e. 43,5 kW.
In the residential area there are just two devices always (mandatorily) running on 3phase: Ranges/cookers (usually 3 x 16 A or about 11kW, wire gauge 2,5mm2) and instant water heaters (correct word?) between 18 kw and 27kW, wire gauge usually 4 to 6 mm2. Use of both is rather common, just I haven't got none of them, because I'm cooking , drying and heating (also water) on gas(NG) actually.
AC is not very necessary (climate), therefore we're depending on models from other markets usually often running on 1 phase with expensive wiring due to uncommon loads.
I think you are pretty right that on small induction motors efficiency is not a real issue, but in the simplest industrial context , 3p becomes indispensable because of better efficiency and much better torque of motors.
The cost thing is not obvious at all. Here we use probably less copper to transport the energy. Prices for breakers and other equipment here are extremely depending on production numbers. A standard B16 A breaker is about 1 to 2 $. A K20 (same brand, same housing, only different electric values) needed maybe for an AC device can be at 20 $. I buy a standard 40A/30mA RCD (sort of GFI) for about 25$, a 63A/30mA RCD is already almost the double price. It means, 3p can become cheaper than 1p in case it is regarded as standard.
The result are rather slim panels. And the price is my work, not the material.
