3 phase ac to 5V 50mA power supply
You can use a three phase rectifier but the problem is that your DC cap will have to be rated for ~500V. Since your output power is just 250mW this will be overkill.



If isolation is not an issue you could add lots of capacitance in series to drop a large part of the voltage before rectifying it. C1, C2 and C5 are AC caps, values to be decided based on load current, with resistors in parallel to drain residual charge in the caps.
C6 is the DC cap and R6 provides the minimum load to ensure that voltage across C6 is always limited. You will also need a fuse in all three phases just in case. 







simulate this circuit &ndash; Schematic created using CircuitLab