3-phase energy meter with CTs
It comes down to how accurate you want it to be. Can you ignore imbalanced 3-phase loads (the assumption being that all three currents are equal and at the same phase angle to their respective phase voltage)?


  I also know of the problem not actually being able to measure the real
  voltage level on each phase


This is the big problem - not being able to measure the real voltage level on each phase. If you don't have a waveform of voltage how will you calculate power - you cannot know the phase relationship between voltage and current i.e. is the load resistive or somewhat reactive.

Another problem is current harmonics - if the applied voltage is a sinewave and the load has significant harmonics at 2F, 3F, 4F etc., these harmonics DO NOT contribute to power consumption BUT they are factored into any RMS measurement of current - how will you deal with these (if any) if you can't multiply (in real time) volts and amps with a decent enough sampling frequency?

If you assume that the load has a fixed phase angle i.e. the resistive and reactive components are always in the same ratio, AND you assume this load is balanced, AND you assume the supply voltage does vary (and you know its RMS value), AND you assume the supply is a sinewave, AND you assume the supply is balanced, AND you assume there are no current harmonics then all you need is a CT and one input.