Criteria for: date_field = ONLY 3rd month from today

Criteria for: date_field = ONLY 3rd month from today

Hi, I am using 
  1. total_for_May = FORM [date_field in 3 months].sum(total_field)
to sum all the total_fields only in May. But this criteria also includes the 1st and 2nd months.

What is the criteria for ONLY the 3rd month?

Also, I would like a solution that only takes up one line of script because I am planning on doing this code 12 times (1 for each month)

I tried 
  1. balance_2month = New_Invoice[Date_Due.getmonth() == zoho.currentdate.addMonth(2)].sum(Balance_Due).round(0);
But this returns null