Combine columns where there is only one data cell per row in Google Sheets

I have Google Sheets data that looks like this:

    A | B | C 
   -----------
1 | a |   |  
2 |   | b | 
3 | c |   |
4 |   |   | d
5 |   | e | 
6 | f |   | 
7 |   |   | g
8 |   | h | 

I need to combine this data into one column like this:

| D |
-----
  a
  b
  c
  d
  e
  f
  g
  h

For every row, there is only one data cell in Columns A, B, or C. Is it possible to do this using a Google Sheets formula?

Thank you all for your help.