yogi_Compute Cumulative Sum Of Entries In Every Third Column Of B1:1

Google Spreadsheet   Post  #2550

Yogi Anand, D.Eng, P.E.      ANAND Enterprises LLC -- Rochester Hills MI     Nov-24-2018

question by: Quxzcover:
https://productforums.google.com/forum/?utm_medium=email&utm_source=best_answer_notification_button#!topic/docs/9sUNBn2PNeQ;context-place=forum/docs

How to Make Formula Only Sum Every Third Column?
I'm currently using this formula: ArrayFormula(IF(Len(I12:12), (sumif(Column($I12:12), "<="&column(I12:12), $I12:12)),)

How would i get it to only look at the numbers in every third column. Here is a spreadsheet showing the problem in more detail:

https://docs.google.com/spreadsheets/d/1qcWnobJOqf6I2fpDSdQkwzUhwLPBc58lQ_TgeMvt3IQ/edit?usp=sharing
Link to where the original formula came from:
https://productforums.google.com/forum/#!topic/docs/m-oqIc2uO7U;context-place=mydiscussions