I understood that a three-phase suppy was three supply voltages and (at the consumer end) neutral (neutral from star transformer, but not delta transformer). If you measure between any two of the three phases, you will get 415v. If you measure between one phase and neutral you will get 240v. Houses (single phase) are connected to one of the phases and neutral - your house may be on a different phase to your neighbour.
E&OE, -steve