Need to chose one data element from 3 columns

Hello,

I have an Excel spreadsheet where I want to populate Column D on my spreadsheet. Column D should be populated by 1) Column A if there is a value, or 2) by Column B if Column A is blank, or 3) by Column C if Column A&B are both blank. Any help is appreciated.

EX:
A B C D
4 4
1 2 1
1 3 6 1
9 9
5 8 5

4 4