I'm trying to validate 3 cells and for some reason I don't seem to get
'validation' to work.
Using cells A1, B1, C1 I want to restrict input if one of the other cells
already has a value, so on cell A1, I validateCustomFormula:

=AND(B1=0,C1=0)

but it does not work. (???)

Could you please give me a light and tell me what I'm doing incorrect?
Any help will be highly appreciated!