how to hide a column in microsoft report designer VS 2008

I have three columns (col1 col2 col3). Col3 shows the results of the comparison between col1 and col2 and if col1 and col2 don't match, col3 will show value don't not match. if all the values matches I need to hide col3.

i tried  iif(col1<>col2>,true,false) - this hides the column if there is one row that don't match.

thanks