Average from the last 3 months - Develop Triangles

Hi All, 

Hope I can help with this. I am working on this project where I need to make the triangle analyses in Power BI, 

And I got a triangle like this:

•  But my next step is to take the average 3, average 6, average 12, and average Hi/lo from this ratio - Let me explain better:
  
  
  • Average 3= AVG from 202207 202206 202205 in Accident month 1 (1.45 +1.56 +1.61/3) = 1.54

                                     AVG from 202206 202205 202204 in accident month 2 (1.06 +1.07+1.09/3) = 1.07

It will look something like this:

How can I take the average from the last 3 months from this development month (columns)??

Please see a sample of my data on Power BI here: 

Here 

Thanks for the help.

Michelle