Create a calculated field to calculate the Average by using below calculated field.
1. Group all orders for last 3 months using datediff().
IF DATEDIFF('month',[Order_Date],TODAY()) <=2 THEN [Order_Date] ELSE ' ' END.
Put the field in filter and unselect space ' ' from the filter.
2. Create calculated field to calculate avg of sales using WINDOW_AVG(SUM([Sales]))
Hope this helps you.