Average of last three Orders

[#1]

I want to graph:

      - Dimension: Month

     - Expression: Average Order size of last three orders in that month.

I came to

Aggr(avg(OrderSize),Order)

Now how do i get it to only calculate the last three Orders of that month?

I read about FirstSortValue, but i am not sure how to use it in this context.

Jarl.