Using Excel 2010, Windows 7

I've got a simple spreadsheet that I'm trying to prevent data entry in two cells if there is any data in a third cell. (Each cell contains a DROP DOWN)

For instance, if there is data entered from the dropdown in D1, then no entry should be allowed in the dropdown from D2 or D3. Or, if data is entered in D2, then no entry allowed in D1 or D3. Etc.

I found the below in your forum, but it's not quite working because it doesn't seem to account for the Drop Down lists in each cell:

Select cells D13
Select from the menu Data\ Data Validation

  • Allow: Custom
  • Formula: =COUNTA($D$1:$D$3)<=1
  • Configure the Error Alert message tab to suit
  • OK

I think I'm close, but can't quite get there.

Thanks in advance. I hope my question makes sense. I'm new.