Voltage is nothing but potential difference, so we can take
only 2 objects if we want to measure the difference between
them. So we always measure this difference of potential
between 2 not 3.

In single phase case; 220 v is potential difference between phase and neutral, whereas in three phase case potential difference between any two phase terminals will be around 415 v. All three lines show a potential difference of aprox 220 v with respect to neutral but are but are apart from each other by an angle of 120 degree (phase difference) so will exhibit almost double potential difference between any two line terminals. So for all practical purposes potential difference between any two phase terminals will be around 415v. However for power calculations etc we multiply 415 with squareroot of 3 to get actual total line voltage which is a sine function of phase angle between them.

that is where frequency does matter, because with frequency one of the three phase become neutral after a timeperiod. so for that reason at a moment two phases are showing 440v in pakistan).