Continuously evolving Sum formula

Is it possible to create a formula or workflow that would move/update our "90 Day total" column as new months come and go? I've attached what we are currently working with. The 90 Day Total is just a sum formula for each Jun Jul and Aug. The rest of the year is mapped in the exact same style, but for Sep Oct Nov and Dec. Once June passes, we want the 90 Day Total column to forget June, and move to July August and Sept instead of Jun July Aug, and so on as the year progresses.

Any help/advice is more than welcome!


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