hi,
two ways I see:
1) use one transformer and 3 optocouplers by which you select phases connected to transformer at the current moment. Probably, to avoid short circuit you sould put 3 resistors in line with optocouplers. Your software should select input phases step-by-step (1-2-3-1...) for a short time (let say 0.33 sec for each one; it depends on power supply capacitors and current loaded). In this case transformer will be connected at least one time per second to working phase and it still should be enough to produce 5V. The big problem is starting of such device if default selected phase is off - you will need to have 3 start push buttons for guarantee start... :(

2) use switching power supply for your device. As a rule, such devices have diode bridge on the input and stabilize yet DC power then pass it to impulse transformer via triak-key etc. So you may put 3 diodes from each phase to that place of switching power supply which has produce input DC.

Good days!