Toddler no longer listens to us?

I know that when a new baby is born kids act out but I need this under control somehow. My daughter is 3 and her brother was born a month ago. When we ask her to do something or tell her to not do something she doesnt listen. This was a kid who was so obedient and behaved so well...now she is becoming intolerable. Today she and I went for a walk and she decided to go and rip out a bunch of flowers when I told her to stay by me. She yanked out of my grip and ruined them. Ive tried time outs, Ive tried calmly talking to her and now I am so frusterated I end up yelling at her which I dont want to do. I need help here!