Im having a mind blank!!

I currently have 3 columns populated with customer acknowledgement dates.
the 1st column is the original date and the 2nd & 3rd are any
re-acknowledgement dates.

What I am trying to do is in column 4 put:

Is column 3 is populted put column 3, if not put column 2, if column 2 is
not populated put colum 1.

Tried using IF & AND but can not get it to work.

any ideas?

TheRook