Why the voltage between phase to neutral is 240v compared to phase to phase is 415v?

What you are describing is a three-phase, four-wire, system having a phase voltage of 240 V and corresponding line voltage of 415 V.

You do not add the three phase voltages (3 x 240 = 720 V) to obtain a line voltage.

The three phase voltages are actually displaced from each other by 120 electrical degrees, so any pair of phase voltages must be vectorially-added to obtain a line voltage. If you perform this calculation, you will find that the line voltage is always 1.732 times larger than the phase voltage. So, 1.732 x 240 = 415 V.