Three phase is pretty unusual in residential areas. It's almost unheard of to a house itself
It is very unusual for a house to have 3 phase. But there's an older development in my town that was built in the late 50s, and the houses had this new feature called "central air". High end houses. I suppose the only equipment that was avaliable to do that job was equipment intended for commercial office buildings, and that needed 240V delta. So they have 208V wild leg inside the house.
Things would get a little "interesting" if a DIYer homeowner there picks the wrong slot of the wrong panel to run a new circuit...
