Connecting DC Common Node (Ground) to Three Phase Neutral
Directly connecting neutral should not be a problem, many single-phase transformerless supplies do that.

This image from atmel app note AVR465 &quot;single phase energy meter&quot;, you may find the remainder of that document interesting too,
A three phase supply made by replicating the capacitor and rectifier part will work in a similar way but with less ripple at the regulator input.