Count of Last 3 Months Unbilled outlets exclude the current Month
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.
