E.S.
I had a similar situation with my 2 boys who are 22 months apart in age. The day the 2nd child was born, my eldest came to meet him and the first thing he did was throw the toy trains that the baby "gave him" at the baby and managed to him him in the head with the trains. Ever since that day he has hit the baby, etc. Time outs and screaming, etc didn't help at all either... and they are both complete angels when they are separate. I think what helped me the most was the book "How to behave so your children will to". The most important lesson from the book was to give lots of praise and attention when the children are behaving well. So be sure to spend more time praising them for playing well together and showing them that you think they are wonderful sisters so that they get the image in their heads that they play well together. Kids really live up to your expectations, so if they think you think they are good and play well together, they are more likely to behave this way. I also noticed that whenever I had a positive attitude and tried really hard to look for good things my eldest was doing (e.g., praise him for playing quietly, etc.), he would behave a lot better.
Also, I think the 3 year old age is the hardest in terms of violence in kids, and when my eldest turned 4 he really settled down and seemed to be less violent. Try not to interpret it too much and think that she will be a bully forever because I can guarantee you that they will grow out of it, especially if you can avoid letting them get the a self-image that they are violent or "bad". Good luck! She will grow out of it eventually (probably by the time the new baby comes), so stay patient and positive with her.