LM2596 voltage regulator not providing steady voltage
Just think about how you have shorted out the diodes in the rectifier stage: -

Consider V1 (red box) for one minute. On one half cycle it will produce a voltage polarity as shown. This will drive current into the ground connection and that current will re-emerge into the main part of the circuit just below U1. Just follow the purple arrows round and you will see that basically V1 voltage is shorted on each half cycle. Ditto V2 and V3.
The problem is that your star point is earthed. Remove it and re-check. Or remove D1, D3 and D5 and live with it being a half wave 3 phase rectifier.
I'm not saying this is the only problem but it's the big glaring problem.