If you are talking about the final transformer for domestic distribution, the the final transformer (UK) is a 3 phase 440 V Y connected. This is so the consumers can be connected one phase and neutral giving 230v to them. In a road the consumers will be spread around the phases so on average the neutral currents balance out. The neutral point is earthed at the transformer, so the neutral voltage is never above 6 V from earth. Inter-transformer distribution can be delta, so saving on a neutral conductor.
In the USA, they use a Y transformer with an additional tap at 110V PER consumer, this gives 220V 3 phase + 110V single phase!!!
Frank