Typically, in my industry, for "accurate" measurements, we use a 3-phase separator and measure each stream separately.
This is the most accurate method that we use.
To measure 2-phase or 3-phase flow using a single instrument, you can get "total mass", but not the ratio of distribution of gas-water-oil.
For 2-phase gas-oil. The best measurement techniqque that we use is a 2-phase separator, and measure each stream separately.
For 2-phase water-oil, we sometimes use a coriolis and a water cut meter. The coriolis tells us total mass flow, and the water cut tells use %water in oil (or % oil in water).
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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There are MANY multiphase flow metres installed in the North Sea. Mostly on remote installations/satellites and sub sea installations. They are not used for "process measuring" but more as a substitute for a test separator.
While none of these can provide fiscal metering some of them are used for partner allocation.