This is the formula to put in M3:
=IF(ISTEXT(Q3),Q3,IF(Q3<>"","Paid",IF(ISBLANK(L3),"",IF(L3<TODAY(),"Overdue",IF(TODAY()>=L3-3,"Becoming Due","Not Due"))))) copied down
You can also use the Custom feature to filter, and select Is greater than or equal to, and enter start date of the month, then select AND and select Is less than or equal to, and enter end date of the month.