Calculate Power of 3 Phase HVAC Unit
If you want apparent power its the sum \$V_aI_a + V_bI_b+V_cI_c\$.  In the USA each phase is 120/208v (relative to ground/relative to neighboring phases).  Assuming you're measuring with say a clamp meter over each phase of the power cable you'd use 120v and not 208 as your V.

If you want real power then you are out of luck unless you can measure phase.  In which case it's the same equations but the above are complex values.