Yes shure, the calculation with “volt * ampere * cos phi = watts” is Ok, but you never know “cos phi” as a const value.
If it’s not important to know precise values this is a good way. Differences maybe sometimes more then ±20% from real Watts!
By the way, cos phi you measured at one place is only valid for devices powerd by the current through this meter.
Devices outside of this current loop, even at the same phase will have totally different cos-phi.
A good measurement would be with real analogue multiplier (OP) for V*A or by sensing both at same time with higher frequency and calculate the pairs of values. And with 3 Phases you need all that 3 times.
That’s why real power meters not very cheap.