Try:

Assume data is in cells A1 to C1

Highlight these cells, select Data Validation, Select CUSTOM and set formula
to:

=COUNTA(A1:C1)=1

Set error message e.g. "Only enter a value in one of the cells A1 to C1"

OK

HTH

"Jane" wrote:

I'm working in a worksheet where I have three cells, in which it is only
allowed to fill in values in one of them.

How do I make a validation rule, which give an error-message, if I'm trying
to fill values in more than one.

Jane