I have a three phase power source which 415V line to line .
and I have three transfomer 240 - 12V.
Now I have a task to step down the three phase system to 12V each phase.
How I can do?
You could take 2 transformers and put the primaries in series. Two 220 volt primaries add up 440 volt. The secondaries can be placed in parallel for 12 volts out.
You would need 6 transformers to do this for all three phases.
Otherwise you need the proper voltage step down transformer.
How many wires your three phase has. If it has 4 wires marked RGB and N then it is a star connection. The voltage between any of R,G,B wrt N is called the phase voltage and will be around 240V for 415v 3 phase system.
Connect the primary of the transofrmer between
R & N
Y & N
B & N
now the load is also distributed equally. The output of each 240-12 step down will give you 12 v as desired.
If you dont have a star connection then you have to have a star-delta tranformer.