T.C.
Right now, with the transition of a new sibling especially, I would be VERY careful of giving her very much negative attention...even if she is acting up. She's at the age where what she is doing is normal anyway, but you don't want her to associate the new baby with her being disciplined a lot, you know?
I agree with Consuelo, make sure you are spending a LOT of time with her still. Whenever we have new babies added to our lot, we include all the other kids as much as possible. They become my little helpers. If I need a diaper, I'll ask them to go grab it - and I make it sound exciting. I try to think up thinks they can do to help me with the baby and make their role as sister/brother be really important.
Often they want to sit by me a lot, even if they can't sit IN my lap. They'll sit next to me, snuggling with my arm or something. I try really, really hard to focus on involving them as much as I possibly can. It would often be easier to not have them as involved, but for their sake, I try really hard to view it all from their point of view as much as I can.
Adding a new baby is a stressful (and exciting!) time. You're likely exhausted and trying to figure out how to make it all work...just take it slow...be REALLY patient with your 3 yr old. She'll likely act up less if she is getting a lot of positive attention from you. Be sure to regularly talk to her about things to do with her and things she likes. During baby's naps, try to do some special activities between the two of you. The more time you put into her in a positive way, the more she will likely behave better overall...I highly suggest trying to back off on negative discipline right now. It really only seems to make it worse! When she gets upset, have a counseling session. As contrary as it might seem, when she gets mad at your or back talks, give her a hug and talk to her, asking her what is wrong and how she is feeling. I know it seems like it won't work, but it does.
It's a tough transition! Just hang in there, and it will get better.