I can't figure out an equation, or if an equation for its even possible. I'm
using Excel 2003

My spreadsheet has multiple columns, but they repeat. A is 'Part number' B
is 'Parts Completed' C is 'Date Completed'.
Then it repeats with C being Part number and so on.

I need an equation that will sum every third column. Basically
=sum(A1+D1+G1) and so on. But is there any way I can make the equation short
so that I don't have to enter in every column? This spreadsheet goes all the
way to column IU or something like that.