Multiple Voltage Switch Mode Power Supply
Building a multiple voltage power supply for an extremely harsh
  environment. Will be in a factory type environment with heavy
  equipment.


I smell problems if you are not isolating incoming supply from outputs. I'd use something like a TEN 30-2412WI from Traco: -


Input 10V to 40V dc
Output 12V dc at 2.5A (30 watts)
Voltage accuracy 1%


The output is substantially isolated from the input supply thus, if you have any factory fault currents passing-by they are not going to fry your electronics. Add a 3V3 and 5V switcher on the output to get the supplies you need.