My mom (yes I know she loves me and all that) but she just doesn't like my girlfriend (of 3 years) at all and she has no reason to and she's giving my girlfriend the hardest time of her life and she's thinking about leaving how can I talk her out of it please help!

How old are you? Do you live with your mom?

I am currently 17 and yes I do

Has your mom told you why she doesn't like your girlfriend?

I think you should sit your mom and girlfriend so you guys can all talk

Has your mom told you why she doesn't like your girlfriend?

Because she supposedly controls me and that I never get to hang out with friends and that she has no rules and that she's one of those girls that is one to stay away from... but it's not like that at all? I don't get it were perfectly fine until my mother comes in and ruins our plans and try's her best to keep me and my girlfriend from seeing each other and making it difficult I really do love this girl and my mothers about to screw everything up :( I don't know why I have to suffer when my mothers the one causing it?

You have to suffer because for now you are dependent on your mother for the roof over your head and all the expenses of living. It's not much different when you are out on your own - some employer will treat you in a way you don't like and you will put up with it for the paycheck, or quit.
You can accept the 'ruined plans' or you can leave home (depending on where you live) and try to be on your own for the sake of this girl. Basically impossible in this day and age.

I'm not going to try to judge you, your girlfriend, or your mom. I hope you stay put and let your girlfriend be the one to decide whether to leave you or accept your mother.

You have far more earth shaking things to be thinking about. What are your plans for your future, for your adult life?
I assume you have one more year of high school?

You will learn that spending your time all with one person, even your partner, is not a healthy relationship. You need your own friends and the ability to have fun without her. If you two were to break up, you would be left alone because all of your friends are finding other people to do things with now that you spend all your time with her. I love my partner very much but we have kept our own friends and activities.