Try this:

=SUMPRODUCT(--(MOD(ROW(A1:A100),3)=0),A1:A100)

Assumes you are trying to add from column A - adjust to suit. It will
add rows 3, 6, 9, 12 etc.

Hope this helps.

Pete

On Sep 19, 3:02*pm, baurian wrote:
Hi,

I need to add every third value in a row of data and I do not know how.

Thank you,

Benedict