I have created a sheet which is tracking 3 items per client and a number of clients so I have set up a SUM function the only way I know how which is a long a1+a4+a7, etc
Its works fine, but if I want to add a new client and 3 new rows, ALL of the + after are now off.

Is there a better way to create a formula that will track say every 3rd cell in a column but then let you lengthen or shorten the column?

Thanks!!