How do you measure 3 phase electrical power?

There are several ways in which you can measure the power of a three-phase load, each of which requires the use of wattmeters, and the techniques vary according to whether the load is balanced or unbalanced. A common method is to use the 'two wattmeter method'. This cannot be explained here, but doing an internet search will provide a detailed explanation.

Another answer

Using a DVM from L1 to L2 or L3. Basically any combination. They should all be equal though, no matter which combination. If they are not, then there is a problem with one of the legs.

Comment

That is incorrect because you cannot use a DVM (digital voltmeter) to measure power.