one way:

Select C6:E6. Choose Data/Validation. Choose Custom from the dropdown,
and enter

=COUNTA($C$6:$E$6)<=1

in the textbox. Enter a prompt/error message, or just click OK.

In article ,
"McKenna" wrote:

I have just come back to excel after a couple of year away, and fustrated
that I can't remember how to do things!

I have three cells C6(provisional),D6(confirmed),E6(rejected), I want to be
able to input a value to one cell, but only if the other two are blank. A
user will need to change the value according to the progression of the order,
but I want the safeguard of not having a value in more than one cell. I've
tried an IF statement, and a IF AND statement, but still can't get it. I
would welcome any help.