Me and my fiancee have been together three years because of certain reasons we have decided not to get married but stay together can I change my last name to his without us getting married
As long as your name change is not meant to defraud anyone or commit criminal acts, you can choose any name you want. I've heard of people changing their name to digits (12345, etc), celebrities (Elvis), deities (God); you can ask the court to change your name to any name you want.
GOOGLE (LEGAL NAME CHANGE -your state-)