The Terrible... Threes?

We have nearly finished the terrible two's with our daughter--she turns 3 in a few weeks! Yet as she approaches this birthday, all kinds of new behavior is popping up, and I am not sure I'm a fan of the 3's. It seems that no matter what age your child is, if you talk to any other parent, their response is bound to be, "You think that's bad? Wait until they are (fill in the blank)." This seems to extend beyond the teen years, so I don't know if we have any light at the end of the tunnel! On a positive note, she has recently become much more helpful, able to do chores, able to undress herself, use the bathroom and wash hands alone, play without me for short periods, and (at times) be very generous and obedient. This is all very exciting, and I enjoy watching our toddler turn into a little girl.

However... then there are the new negative behaviors. We have just started the delightful "why?" stage, and the endless chatter and questions is driving me crazy! Additionally, instead of 2-year-old tantrums, she has now become defiant through back talk "I don't want to!" or "I'm too busy!" and through physical violence. Most disturbingly, she has become downright abusive of her little brother, who is not quite 1 and 1/2. Despite our constant efforts to teach her to be gentle and talk to him, she pushes him away, takes his toys, hits him, yells at him, and even sometimes bites him. Of course this horrifies me and we have been trying to discipline her! The problem is, time-outs, slapping her hand, and even strong spankings on her bottom don't seem to have an effect on her behavior. Sure, she might cry or act remorseful for a few minutes, but then she often repeats the behavior as soon as she comes out of her room or the time-out chair. At least she doesn't act this way around other children--just her brother. And it is worst when Daddy is not around for a little while. Unfortunately, with a deployment coming up and another baby on the way in a few months, I am becoming incredibly nervous about my ability to control her and keep her little brothers safe. It's sad that she is such a happy, energetic child, yet shows such ugly, vicious behavior! Does anyone have advice on dealing with defiant/stubborn 3-year-olds? Or discipline ideas besides time-out and hitting? We are considering reward stickers or a chart for good behavior, but aren't sure if she is old enough for that...