Count of Last 3 Months Unbilled outlets exclude the current Month

[Priya_10]

I have a column named "flag" in which "1" is against billed outlets and "0" is against unbilled outlets. I want to calculate the count of outlets that are unbilled from last 3 Months and exclude the current month. 

In full data: year_month range  is January 2018 to Decemeber 2020. 

In Sample data provided for help: year_month range is 2020.

Attached sample data image. Please guide me in using the DAX expression. 

[lbendlin]

Please provide sample data in usable format (not as a picture) , show the expected outcome, and give sufficient context.