Allison:
First off I'm sorry you are going through this.
Removing her from the situation is the way to go. At age 3 she should be able to express herself WITHOUT violence.
Once she is removed from the situation - ask her WHY she did that.
Does she WANT to hurt others? if so - WHY? Keep it simple - do NOT overload her with questions - but at 3 she SHOULD be talking and using her words instead of her hands.
You may be consistent with removing her - but are you consistent with punishment as well - when she won't listen, hits her brother, etc. does she have toys or privileges taken away from her? If not - make it consistent - if she can't keep her hands to herself - then she cannot have anything that she can hurt others with.
I would also enroll her in a martial arts class - I know - I know - I know - but even at 3 - the Masters will work with her to instill self-control, etc. try it and see. She may be on the HIGH end of hyperactivity and her brain is going 1000 miles a minute when everyone else's is going 10 miles a minute.
I would DEFINITELY call my pediatrician TODAY to set up an appointment ASAP to have her evaluated.