ISBLANK in 3 Rows Conditional DATA Validation EASY

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.

You guys are awesome thanks!!!! I'll try that second one tomorrow....