3 1/2 yr old acting up since baby was born!!!?

I have a nephew who is three and he will say "I am mad". I will ask why and he will tell me. Talk. I always talk. I am not saying not to punish her when she acts up. Like put her in corner for a 3 minute time out and then ask her why she did what she did. Children are smarter than what we think. My nephew was mad at me because I did play the right way. It was a big deal to him but not to me. We talked, hugged and played. If she does not want to eat then don't but let her know that dinner time is now and you sit at the table with mommy and daddy. It will get better. I know you just want it to happen now or very soon but it may take some time. Communication, I think, is big even at a young age. I truly hope any advice that we gave you will help. Take care.