How is a house wired 3 phase?

Houses are not wired for three phase power as there is typically no need for that much power. 120/240 volts is the typical home voltage. Even some vary large houses (mansions) are still the same single phase voltage, just a very large amperage service (voltage times amperage is wattage, and use is rated in wattage). Utilities charge a base rate for homes that is usually in the range of $10-$30 per month, this is the charge when no electricity is used. For a three phase service, the charge is typically $125-$250 (depending on location and utility) per month before any power is used. Most all wires installed in a house are single phase. The black and white in 2 conductor with ground NM cable are suitable for only two of the three phases. 3 conductor with ground can be used, but that is not installed unless there is a purpose for the third wire that makes it unusable for the third phase. Three phase power must be designed into the construction of the house before it can be considered for a service, and then the cost is likely too high for the average owner.

Alternative Answer

The above answer applies to North America. But in other parts of the world, three-phase supplies to residences are quite common. For example, my house, here, in Cyprus, has a three-phase service. In these cases, the consumer panel has three separate line busbars, and the residence's loads are balanced between each line.