You could use 3 transformers, which each will give you a fully floating low AC out, so you can connect them to a earthed control circuit. You could use opto isolators with a series resistor and a reverse protection diode, this will be cheaper then the transformer solution BUT will need some sort of gain control to calibrate and balance their outputs. Make sure that the output from either system is clamped with diodes and has a series resistor and has adequate clearance from other components on the PCB (a completely different board would be better). This is to ensure that your monitoring system will never be connected to the three phase in the event of an insulation failure.
Frank