Here is a simple problem... it should be a no-brainer but I just can't get it to work.

I have column A, B and C. I want Data to be entered in A only when B and C are blank. Same with each column. Data can only be entered in B when A and C are blank and so on... (so that Data can only be entered in one Column at a time)

I use Data Validation : Custom

Formula =ISBLANK(B1:C1)

as such it works, but when I perform the same operation for columns B and C, then I can never enter Data without getting my custom error msg, even if the two other cells (B1 and C1) are blank


More details: basically this would be for budgets and some amounts can be put in weekly, monthly and yearly... but two timeframes cannot be used for one item.