480V 3-Phase AC to DC 12V
I am trying to figure out the best approach to convert a 480V 3-phase AC to a 12V DC. I am thinking of first converting the 3 phase to a single phase AC ...


You cannot convert three-phase to single phase using transformers other than by dropping two of the phases as you suggest in your last paragraph.


  To Convert a 3 phase to single phase can we use one of the phases and a neutral wire in a 3-phase 4-wire system and give it as an input to the above transformer and rectifier or is this a bad idea?


Yes, it's a bad idea. It puts all the load on one phase when you could have a three-phase balanced load.


  Is this the right approach or can anyone suggest better approach?


A very simple and economical approach is to step the voltage down with a 3-phase transformer and rectify it.



Figure 1. 3-phase rectifier. Source: if a standard three-phase 400V AC connection is rectified what DC voltage comes out of it?.

The DC voltage obtained will be very close (about 95%) to the transformer peak output voltage less the diode voltage drops. Therefore to get 12 V out the transformer secondary voltage should be: 
$$ V_{IN} = \frac {1}{\sqrt 2} (12 + 2 \times 0.7) = 9.5\ \mathrm V $$



Figure 2. 3-phase transformers are available in sizes from 100 VA or so up to magawatts.

DC power supplies are available similar to that of Figure 2 complete with built-in rectifiers.