If Date_Vsn_Sourc_New_Mem_Pgm is the date that program began, try something like this:
sum ({<[FISCAL_YEAR_NUMBER]={'$(vCurr_Yr)'},[VS_New_Member_Program]= {'Y'},[PROD GROUP] = {'ABCD','ABCD','ABCD'},[TYPE] = {'AB'},INVOICE_DATE={">=Date_Vsn_Sourc_New_Mem_Pgm<=$(=Monthend(=AddMonths(Date_Vsn_Sourc_New_Mem_Pgm,3)))"}>}QTY))
Not tested
