my three year old is throwing a fit all the time, not lissoning screaming hitting herself. what can i do.

listening.

is she trying to get your attention?

How do you react to her when she is throwing a fit?
First, we must understand that toddlers are going to throw fits. It's normal, and they cannot be totally prevented.
We must understand WHY toddlers throw fits. A lot of times it's because they are frustrated by something they want to do or want to have but cannot. Understanding this allows us to approach our toddlers with compassion and understanding. Being calm and patient works much better than being upset and frustrated.
Give your toddler a hug. Explain to her calmly why she cannot have (whatever it may be) or help her do something she is trying to do and encourage her to keep trying. Finally, redirect you toddler from whatever is making her upset to something that will make her happy.

She sounds overwhelmed, full of bad feelings that she instinctively knows she needs to let out.
It's very hard to "hear" that & keep it together. So it makes sense that you are struggling, too.
Think of her system as overwhelmed with frustration & sadness over the things that are not going her way. If you are trying to explain "Why" to her all the time, that adds to the frustration & fury.
Understanding why she's upset (as LudvikSmith says), or actually just getting why it makes sense that she's upset, is important because in this internal space of understanding & caring, YOU have the emotional regulation she lacks at the moment. You can kind of "shepherd" her through the upset to a return to equilibrium.

Chances are you are mostly trying to STOP the behavior, either through disapproving/getting upset, ignoring her, or explaining/trying to reason. Try focusing on understanding how it makes sense for her to be upset, instead.

And what I mean by that, "try focusing on understanding how it makes sense for her to be upset," is essentially a way of responding to a tantrum. If she is screaming about a limit you're holding, something you're saying No to or something she wanted that can't happen, instead of focusing on trying to stop her tantrum/screaming, recognize that she is working to adapt to an uncomfortable limit & that there are a lot of feelings of frustration involved. She is powerless, essentially has to rely on being understood & managing to convince an adult if she wants things to go her way. She made her point to you but things still didn't go her way! There's sadness with that, a feeling of loss & grief, and being angry is safer than feeling her powerlessness & helplessness, plus her grief, so fully. So she rages.
Keep her safe, keep yourself safe, but stay understanding. It's a process. They learn to handle emotions when WE handle them!