How to count every third column

[mellyh]

Hi there, I am trying to count entries in every 3rd column, but it is not a SUM - I have date columns every 3rd column and I need to know how many of them are completed - there is other information in the other columns but I don't want to include that in the count - can someone please help.
Thank you

 

[Scott Huish]

Try this:

=SUMPRODUCT(--(MOD(COLUMN(A1:Z1)-COLUMN(A1),3)=0),--ISNUMBER(A1:Z1))