Hi J.
Your question has nothing to do with conditional formatting (currently called Conditional Highlighting.
Briefly put, conditional highlighting is about changing the style or colour foa type or the background if specifiic conditions are met. The rules compare the value in the cell to be formatted with a fixed value, written into the rule, or with the value in another cell in the same document. Your question asks about placing a specific value into a cell there is any content in another cell. That's not format.
To your question:
Your description of what you want is ambiguous.
You have three cells whose content or lack of content is to control the content of E2.
There are two possible states for each of three cells:
B can be empty or contain 'something'
D can be empty or contain something
C can contain A or B
2x2x2 gives a total eight possible states for the set of three cells, each with two possible states.
Here is a table showing the eight possible states of cell in the same row of those three columns. What is your expected result for column E in each row from 1 to 8?

Regards
,Barry