Preventing Duplicates

I have used data validation to prevent duplicate values by using:

=countif($a$1:$o$20,a1)=1


but now I'm forced to prevent duplicate values that may occur in only three columns.

I tried without success to paste previous validation into only the three columns, but it's not working.. The three columns together could not contain contain any repeated values.

I suspect I am going to be forced to use VBA here..

Any ideas...

Hi

which 3 columns?

rylo