You would measure the voltage by rectifying the low voltage from the potential dividers or small transformers. You would measure the current by rectifying the output of the current transformers on the output of the power transformer. You would probably need to use active rectifiers using op amps rather than just using diodes. (You may also want to add circuitry to give true RMS output. You will find circuits to do this if you Google "true rms rectifier") Once you have the AC voltage and current represented by DC voltages you would measure them using either the internal ADC which is available on most micro controller families or an external ADC. You could drive a display (LED or LCD) directly from the micro controller or send the data via a serial link or Blutooth to a PC for further processing and display. Chose one of the micro controllers that you are most familiar with programming.
Overnight I thought of another approach to your problem. If you had a phase locked three phase sine wave inverter you could compare the instantaneous voltages on each phase between the power transformer output and the output of the inverter
Les.