Hi KG,
If I understand you correctly there may be several options.
1.
If you need to know the voltage level for each phase, then you could use 3 individual single phase transformers,(e.g. 220V/12V), and connect their primary windings in a Y configuration thus creating your own artificial neutral point. Now you can connect the secondaries of the 3 transformers also in Y, again creating your own low voltage side artificial neutral/common point. The advantage here is that you have 3 isolated low level voltages which you can now use to measure the absolute value of the 3 phases. You can also rectify these 3 voltages to provide a DC source with lower ripple and as long as 1 phase is still working you have power for your electronics which you back up with a battery anyway.

2.
If the absolute value of the voltage does not need to be monitored and the only information required is to know whether the voltage is above or below a certain level, then a simple voltage devider, diac/zener, and optical isolator circuit can be constructed per phase. Once again an artificial neutral can be created by connecting these 3 circuits in a Y configuration. Now you still have your isolation and 1 logic level per phase to tell you if your supply voltage for each phase is above or below your requirement.

If 1 phase to the distribution transformer is lost, then a much lower voltage will still be induced into the secondary of the lost phase due to flux linkages in the 3 phase transformer. This induced voltage is also influenced by the load in the other 2 phases.


Cheers,
Tony