Transformers are normally wound as either single or three phase units for connection to a single or three phase electrical supply respectively. A single phase transformer requires a single live (phase) connection and a neutral to the primary for operation. A three phase unit requires three lives (phases) and sometimes a neutral. The phase to phase voltage for a 3 phase plus neutral system is 1.732 x Phase to neutral voltage.
E.g. A 415 V 3 phase supply has a phase to neutral voltage of 415 V / 1.732 = 240 V
Note: 1.732 is the square root of 3, to 4 significant figures.