Get 3 transformers and connect them between a phase and neutral; in this case transformers will be rated for standard mains voltage, which is ≈230V ..
For each channel you will need a zero-crossing detector for phase measurement, and a peak detector for voltage measurement ..
Outputs from zero-crossing detectors can be fed directly to a microcontroller ..
Outputs from peak detectors have to be A/D converted, so you can use a microcontroller with build-in ADC ..
Examples of zero-crossing and peak detectors are below ..
Rest is just software .. but as you haven't indicated type of microcontroller, there is no point of elaborating on the software side of this project ..
Regards,
IanP