Hi,
To be able to calculate the phase shift you need the zero cross of voltage as well as zero cross of current.
Please consider:
If you have symmetric load you need just one pair. Otherwise you need a pair for each phase.
Please know that: cos(phi) = power_factor is only true for purly sinusoidal waveforms.
For varying currents (sometimes small, sometimes high) it's very difficult to get a precise zero cross signal.
Also note that overtones or other noise in voltage and current will cause the phase shift to jitter causing faulty results.
Thus: if you have varying currents and/or overtones in current, then I recommend to use 4 (1 x V, 3 x I) ADC channels to measure and calculate V_RMS, I_RMS, phase shift, true_power, power_factor .... precisely.
If done precisiin measurements many times. If you want precision your task becomes really difficult.
Klaus