Phase voltage vs main voltage in three phase system
You can use either but you have to remember that the phase to phase voltage is \$ \sqrt{3} \$ times the phase to neutral voltage.



Figure 1. The phasor 3-phase and neutral diagram.

I find the simplest way to remember is to use the phase-neutral voltage if you have it:

$$ P = V_{P-N} I $$

If you have the phase-phase voltage then you need to divide:

$$ P = \frac {1}{\sqrt{3}}{V_{P-N} I} $$

The \$ \sqrt{3} \$ term just comes from the trigonometric relationship between the voltages in Figure 1.