J.S.
I agree that 3 year olds are self-centered(not in a bad way), they just don't realize that everybody else has feelings and not everything goes their way. You need to help her see that she is hurting her brother. If she really adores him as you say, then help her to see his pain. Then have her sit in time out. Afterwards talk to her on her level about throwing toys. The time out is not just to sit in a corner or chair. It is for the child to think and to be removed from the situation (while you calm down). I also take toys away when they are thrown for the rest of the day. If the child proves them self worthy the rest of the day, then I make a big deal about it that night bringing the toy back out of time out. If I bring a toy out of time out, I usually do it before bedtime (unless the time out occurred after dinner) because they will forget why the toy was in time out in the first place after going to bed. I also don't allow my older child to play with the baby alone, for a couple of hours, to make sure that it doesn't happen again. If it is not handled correctly, your older child will be resentful of the younger child. I am not sure I handled this correctly with my oldest two kids (11 & 8 now) when they were little, my older one still hates the younger one and it is an ongoing battle. Definitely, keep it about the hitting and not the child. Good luck