2 separate columns with dropdowns using the same source from another dropdown column

I wonder if any of you can help.
Column A is a dropdown list of departments PCS PKS DIS
Column B is a dropdown list of sub departments of column A PCS = AL, BL or CL; PKS = BA, BT, BX; DIS = LA, LT, LX
Column C is a dropdown of categories based on the Column A list PCS, PKS, ACL ..... 10 - 20 entries in each list.

The problem I am having is that I have set the departments to the categories using data validation but when I want to select the sub departments it obviously wants to give me the list of the categories.
How can I get the dropdown list for column B (sub departments) to show based on the data in column A without impacting column C

I would like to add an attachment but it won't let me2018-02-23_14-35-56.png
as you can see I have an extensive name manager (this is after I have stripped it out of confidential data).

If you can let me know how I can share an excel file on here it may help

Thanks JeteMc,
yes that is where I started from and was able to build my table with multi dependent dropdowns however what this doesn't cover is when 2 dropdown columns rely on the output from a same separate column.
Fortunately I was able to do it by using a combination of INDIRECT for one column and VLOOKUP for the other ........ Thanks for letting me know the paperclip doesn't work for future posts.... we can mark this thread as closed