Measuring 3-phase power. How to calculate?
The simplest way is probably this; the two wattmeter method: -
Image from Two Wattmeter Method of Power Measurement and, I mention this because, it is a true 3-phase measurement of power into a 3-phase load. There is also the three wattmeter method: -
Image from Measurement of Three Phase Power: Three Wattmeter Method and, again I'm showing old fashioned wattmeter methods because, if you want accuracy despite non-linear loads or asymmetrical voltage supplies you have to look at what a wattmeter does.
A wattmeter is an analogue multiplier of the voltage and current waveforms. In other words, it's \$v \times i\$ with no worries about power factor. If you try and calculate power using power factor then you have to measure phase angle between \$v\$ and \$i\$ and, the massive problem with this, is that a typical load may be somewhat non-linear (i.e. not wholly resistive) therefore, using zero-crossing detectors (for example) for estimating phase angle, is a bit useless and, gives rise to significant errors.
In other words, your formula assumes a balanced and linear load with a voltage source that is balanced both in amplitude and 120° phase angles. It's folly to think you might be able to do this for practical loads except a kettle and, I don't think they make three phase kettles. Sure there are 3-phase heating elements and these'll be OK but, in the main, you'll be disappointed with V.I.Cos(φ).
But, if your supply is both amplitude and phase balanced and feeds a linear load then you can use that method.
Going back to \$v \times i\$, you can employ modern digital techniques and sample at around 20 times the line frequency as a minimum to avoid current waveform harmonics contributing too much of an error. If you sample and multiply at a higher rate then even better.
Then you accumulate the \$v \times i\$ sampled value over (say for instance) 1 second and divide by the samples per second. Use this method in the equivalent of a two or three wattmeter method.