SharePoint Designer 2013 - Steps to return a unique value if item is found in different columns

I am trying to have some leads on how I can implement the following in SharePoint Designer, but failing so far to find possible solutions.

Basically, I want to look up the value of Column A for a specific row in Column B and Column C (any row), if this value is found in either column, I want to assign the same unique identifier in new Column D to all columns where that matches this criteria.

E.g. value of Column A in row 5 matches with value of Column B in row 3, then I want to assign a unique ID let’s say ‘123’ that will be present in Column D, for both row 5 and row 3.

I hope this clarifies and many thanks in advance for your support.