Excel Excel: Data validation w/ ISBLANK 3 Columns

[skillzflux]

Good day all, I have a simple problem... but just can't wrap my head around it.

I have columns A, B and C. I want data to be entered in only one column at a time with the ability to change column as you go down rows, but never be able to enter data in two columns on the same row.

I use Data Validation - CUSTOM

Formula: =ISBLANK(B1:C1)

and it works great for that one Column... but when I repeat the same operation for the two other columns everything falls apart. I get an error msg upon entering data in A even if B and C are blank.

If I do for two columns, no problem it works perfect but that third column breaks everything as if all the conditions nullify one another when applied to all 3 columns.

any ideas?
please and thank you!

 

[skillzflux]

Solution:

=COUNTA($A1:$C1)=1