How to get 3 month rolling average from aggregated calculation

I am trying to get a rolling 3 month average score within my results using an aggregated calculation. The calc is the difference between two percentages: i.e. sum(producta) / count(producta) less sum(productb) / count(productb) = 85% less 10% = 75%

I have this result each month but I now need to have a rolling 3 month average however I must use the formula above for the 3 months together rather than just avg each month. Refer to picture.

So in the end I would have a constant 3 month avg for each latest month including that month:

i.e. Aug2020 78.24 (Jun, Jul & Aug)

Jul2020 xx.xx (May, Jun & Jul)

etc

Thanks for any help.