K.S.
Is it garnering extra attention? With a new baby in the family...That seems like reason enough for a 3 year old to do all sorts of wacky stuff. Are you using time-outs for this behavior? I would try a time out (I put a backless stool in the middle of the kitchen and make them take the time-out there) and tell her she can say a nice hello/good-bye when she is ready.