Crystal Reports 3 Month Trailing

Message

   I have my report grouped by months.  There is summary thats add the total of a field by month.
I want to add the previous two month's totals to the current month's totals.

Currently Shows:
December 150
January       50
February     75
March       100
April         100

Want It To Show:
December 150 (October+November+150)
January    200 (November+150+50)
February  275 (150+50+75)
March       225 (50+75+100)
April         275 (75+100+100)

Edited by WhenF1CantHelp - 06 Aug 2007 at 3:20pm