Hi K., I am sorry I'm responding to this late. I have a three year old daughter, too, with the execption, of I have three older boys. She is also into throwing items, or even hitting the boys. I have found a solution, and I hope this one works. I have tried the time out thing, with no prevail. I have tried the sticker thing, and that did not work either. I finnally sat down with her, like a big girl, and asked her why she was doing it. She told me it's because they tease her and make her feel like a baby. She does not like being treated like one. So, we have enrolled her into Preschool. She goes three times a week for three hours a day. She feels she is a big girl going to school. That may be another idea for you. Just see what the preschools in your area say. I know when my second boy was born, my oldest was a bit jealous, but he realized after awhile, that mommy takes care of baby, then me. I get more time with mommy than any body else. Just encourage your daughter, that when the kids go home, you have more time with her. If she does not take naps, and the other kids do, spend some time with your three year old. Let her help you with chores at that time, and explain to her, that the other kids can not help you, only she can. Maybe she will come out of it.
Hope all goes well with this.