Tell her that you are hurt and angry and why you are so hurt at her dismissal of three years of love and support. She has decided that she would prefer a mono relationship - that happens and she cannot be blamed for wanting what she feels is best for her. But that does not mean she gets to rewrite history, pretend you and she never happened. Tell her you will not hide the truth from future boyfriends. You can't stop her from lying or shading the truth but do not help her do so. If they ask, you will tell.
Also start thinking about how you will handle it if her relationship fails - and the odds on new relationships tend towards failure. Do not be surprised if she wants to come back to you. How will you handle that?
Best of luck.