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

Month total avg3monthstotal
Month1 50
month2 30
month3 56
month4   47
.        .
.        .
.        .
Month12 56

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