Hi there. My thoughts are that perhaps you are making too big of a deal out of, the pregnancy to her. She wants to feel number one. I'd just not really make a big deal about being pregnant and plan some extra special things for her that are all about her and nothing to do with a baby, buy a gift to give her when you have the baby telling her it is FROM the baby, tell baby that "you have to hold on while I take care of sister" (baby won't care, do this at a moment in which baby is content) so that when you tell your three year old that she needs to wait a minute (so she feels everyone takes a turn at waiting), have two boxes you rotate that you bring out one when nursing with puzzles, books, toys that she otherwise doesn't get to play with so she looks forward to your nursing so she can play with the toys, etc.
I had a toddler and gave birth and I did these things (my pediatrician recommended them) and we never had any big issues.
When I brought the baby home, I just placed him on the floor in his car carrier and showered my other son with attention. Rather than being super big on introducing them straight away---- then I gave my son his gift from brother --- he opened it, looked at baby up and down and moved on. No big deal, baby is in the house and he comes with presents! :>)
You might also want to explore buying, "hands are not for hitting", or any of the other books on anger listed on this link. http://www.amazon.com/Hands-Hitting-Ages-Best-Behavior/dp/1575420775
This is part of a series of books aimed at 4 to 7 year olds and meant to be read to them at night (several times) and then practiced. Kids do need to be taught how to deal with anger.
Thank you. Will definitely look i to those books. This issue is just an every now and thwn thing and never hitting anyone else other than myself. Thank you again.