This calculation is done based on the general formula PF=P/S, where P - watts, S=V×I - volt-amperes.
However, a 3-PH circuit requires an additional explanation. Generally, we have to add up "P" and "S" values from all phases, where SPH=VPH×IPH. What complicate matters is that Wye configuration is usually referred to by its line-to-line voltage, which is VL-L= √3×VPH. Conversely, for delta you usually know line current, which is ILINE=√3×Iph. Fortunately, for balanced loads in both cases the total apparent power can be described by the same equation: S=3×Vph×Iph=√3×VL-L×IL .
Therefore to avoid confusion and to use a common approach, for 3ph configuration our widget uses line voltages and currents- that's what you need to enter into respective fields for RMS volts and amps. For sizing of power factor correction capacitors, see our PFC capacitor sizing.