With data validation perhaps. Assuming the 3 cells in question are A1 - A3,
select them and then

Data|validation|Settings|Custom

and use the formula

=COUNTA(A$1:A$3)<=1

Mike

"T.Lindholm" wrote:

I have three data cells in my table that I need to check for data. If one of
the three cell has data in it, the other two can not be filled with anything,
somekind a pop up should be good too. I hope someone understand what I mean,
and can also help me out.