Suppose your 3 cells were A1, A2 & A3.
Use
=datevalue(concatenate(A1 & "/" & A2 & "/" & A3))
This turns the text into a date number. It can then be changed with Format -
Cell -Date and the date system you want to use.

Hope this helps

"M&A_Jack" wrote:

I have a date format written as "31/1/2005", when I concatenate after I've
separated text to columns I can't change format to 31-Jan-2005.