Average of columns Help Needed

[Bruce Smith]

I have a table that records the forecasted orders in a month
Each month for each product a new row is created.

ForecastID SalesSKU MonthNo Date Quantity
1 ABC101 1 01/01/2017 300
2 ABC101 2 01/02/2017 750
3 ABC101 3 01/03/2017 800
4 ABC101 4 01/04/2017 550
5 ABC101 5 01/05/2017 600
6 ABC101 6 01/06/2017 450
7 ABC101 7 01/07/2017 280
8 ABC101 8 01/08/2017 370
9 ABC101 9 01/09/2017 120
10 ABC101 10 01/10/2017 550
11 ABC101 11 01/11/2017 600
12 ABC101 12 01/12/2017 800
13 ABC101 13 01/01/2018 500


I want to add a column to the above table that averages out the current month, the previous 2 months and the next 2 months. I appreciate that the formula would be different for monthNo 1 and 2 but does anyone have a formula that would work for all months afer month 1 and 2?
Happy also with a view that does the same. I just need to display the averages and be able to relate that back to a salesSKU, a month and a year.
Any help would be appreciated