I see. Dare one ask, for what reason this is done, other than possible installation costs of main cabling by the suppliers and (total guess) a way of providing backup should one of the "wires" fail?
Its about efficency, not redundancy.... three phases is an efficent number to generate (and its also an efficent number to use to run motors), its also more efficent to distribute... if I need a 6kw heater... if it was single phase, I'd need two conductors in the conduit each capable of carrying 26A, if I got a three phase one... I'd need 3 capable of carrying 8.7A each (or 4 if I needed a neutral as well)... this plays out at all sections of the network