Look up 'Blondel's Theorem'.
Roughly: to meter the power delivered by a system of N conductors, you need to make N-1 current measurements and N-1 voltage measurements.
In your case, you have 3 conductors delivering power. You measure the current on 2 of them, and you measure voltage between those 2 and the third, and you can calculate all power delivered by the 3 conductors.
Additionally, with 2 current measurements you can calculate the current on the third. Similarly with voltage measurements between any two pairs you can calculate the voltage difference on the third pair.
-Jon
