using multiple buck step-down voltage regulator modules to convert 50-400V DC to 12V DC
Those regulators are not designed for such high-power applications.  You can't connect them serially because they were not made to be poly-phase.  If you try to connect them in this manner, the feedback loop in the converter will freak out and can cause very unexpected results which may damage whatever you are trying to connect it to.

Your required large input voltage range poses kind of a problem.  I'm having a hard time finding a converter that can accept up to 400V.  

Regardless, you are going to need a much more suitable solution.  The converters you have are not fit for this in any way.  You will want to be looking for buck switching mode power supplies

EDIT: Generally you would convert the 3-phase AC down using a transformer.  Common DC voltages output by the transformers are around 24v, 12v, among others and this approach is much more easily usable by other switching mode power supplies.