I'm probably going to play devil's advocate here (based on the first commenter.) I completely understand where you're coming from. You're the one being abused, you need the support, etc. I've been there, too. But I've also been on the other side...You can NOT shit talk your parents, vent about them, complain about them, etc. and expect her to just be supportive, understanding, and a great listener - this happens when she first realizes what's going on. But after 3 years?! She knows your parents are abusive, she knows what they do to you, she knows how they make you feel, she knows that after a really bad fight, it can bring down your whole week, and it happens over and over again. She knows how it makes you depressed, how it's probably the #1 topic, how this and how that and etc.
Has she been pressuring moving in together? You moving out? Or at least you standing up to them? Is that an option? When you really love someone, it seriously hurts to see them get abused over and over again, and you stick around because it's an easy commute? Think about how, the person that loves you, is dealing with knowing you get shit on, on a daily basis, and you just take it? Of course she's frustrated when you try to tell her about how abusive your parents are and how you grew up being abused and then you walk into their house for dinner every day!
Having been there, I know how easy it is to ask yourself why your SO is getting mad at YOU for being abused. Why is she playing the victim? Please see this from both sides. I was stuck in abusive relationships (verbal and physical) in the past, and you're complaining about the wrong thing here and asking the wrong damn question. You should be asking yourself, how can I move out, away from my toxic parents that are hurting me and frustrating my girlfriend because she's had to watch it happen for YEARS?