I agree she needs extra attention, she should have one to one time maybe dad can help more with that . She has realised that bad behavior gets attention..You ask about pullups well I dont think its a good idea it is reinforcing the behavior you are trying to stop and allowing her to wet herself .She is feeling left out, walk in her shoes ,it will pass its very common .
I have heard of this happening with the appearance of a new baby. The regression could be due to the fact that the majority of attention is going to the baby. She is used to being the center of attention, so now she is making the demand for it. My friend set aside special time for just her & the oldest where they did whatever she wanted to do & told her that it was for being such a great big sister. This made her feel that just having a baby sister made her special & the behavior stoped. I hope this helps. Best of luck.
This comes up here all the time, so know she is not alone.
I personally think there are two different possibilites. First one - totally not uncommon for 3 year olds to still have accidents. Many of them just don't have full control yet, and even if she did well for a bit, regressing at this age is really nothing to be overly concerned about. Secondly, a new baby is a big change in her life. I cannot tell you how many moms have written in here saying the same thing happened when a sibling came into the picture.
She is still so young. I wouldn't make a fuss at all about it. Put her back into pull ups and give it some time. I suspect it won't take long until she is back on track. Honestly, not an issue, at least I don't believe it is. Best of luck!