Individual Three Phase Potential Transformers vs Three Potential Transformers in one case
There are four options:


A three-phase VT wound on a three-limb core.
A three-phase VT wound on a five-limb core.
Three separate single-phase VT's.
A voltage measuring device that measures your 600VAC directly without a VT. (Where I come from, direct voltage meaurement, without a VT, is used up to at least 440 VAC.)


Of these, I would recommend that you avoid VT's wound on a three-limb core, as they are not capable of reproducing residual voltages. This is because there is no magnetic path in a three-limb core for any residual flux to flow.

That is, if your voltages VA + VB + VC do not balance to zero, a three-limb VT will not reflect this imbalance. This sounds like it is critical for your application.

The extracts below are from a typical drawing I prepared for HV directional overcurrent/earth fault protection.