How to Sum last three groups

I have a report that has a Payment field on the details line. It is then grouped by Month. The group footer contains a sum of the payments for that month.

How do I create a formula that displays the sum for the last three months?

Mon Sum 3monthSum
Jan 20.00 20.00 //(Jan)
Feb 10.00 30.00 //(Jan - Feb)
Mar 40.00 70.00 //(Jan - Mar)
Apr 30.00 80.00 //(Feb - Apr)
May 15.00 85.00 //(Mar - May)

Thanks,Do you want to find summation of sum of each month?
Make use of Running total feature