Last 3 month sales value in Script

[Anonymous]

All Experts,

Need your help in calculating the last 3 full months sum in the script not in the front end.

There may be more than one transaction in a month and I want to sum all the previous three months(excluding the current month)

Sample data with the expected output attached for your kind reference

Any help would be much appreciated

Regards

Jeba

[renuka_sasikumar]

Hello, My requirement is the same, but here I have 

CALMONTH&'-'&CUSTOMER&'-'&MATERIAL AS SalesKey

Can you help me with the script.

Thanks in advance