Easy solution, You can put 3 OR conditions and add in the date range from 1 to 20th. A better solution is to add in the last 3 months. And add another condition to get the day from trandate and see if less than 21.

But how can we define this 21 ? This should also take month along with 21.

you can use a numeric formula field, convert the date to just day and criteria between 1 and 20 and filter invoices for last 3 months. Group them by Month on results