3 months average from running total

I have existing rolling 12 months running total report...

Month    total       avg3monthstotal

Feb-15    4

Mar-15    6

Apr-15     5

May-15    4

June-15   3

July-15   4

Aug-15    4

Sep-15    7

Oct-15    5

Nov-15    4

Dec-15    4

Jan-16    5

Feb-16    4

How do i get the average 3 months values for each month like..(month + month-6 + month-12) ?

I would appreciate if someone can help me with this.

Thanks