Three phase 230V input to single phase 5V/5W power supply
SMPS operated by AC normally first do rectification of the input voltage, i.e. they work also with DC or pulsating supply voltage. 

So you can simply connect all three phases by diodes and use that as input to such a SMPS.







simulate this circuit &ndash; Schematic created using CircuitLab

added note:
But make sure you use such a SMPS. 
It will definitely NOT work with a simple 50/60Hz transformer supply.