I hate to laugh, because I know its rough the first few times, but my oldest (nearly 5) tells me she doesnt like me on a daily basis. Im the "worst mom on the face of the universe" at least 3 times a day. I just tell her that means Im doing something right now. It used to bother me, and now Ive just come to realize that kids have no idea what theyre saying.
Your little guy is only 3. He probably doesnt quite understand yet. Give him time. He'll come around. Its going to be a big adjustment for him. The more involved you can get him the better! Have him help pick baby things, paint the nursery, put together furniture. Whatever. It helps. When we had #2, I had a special gift packed in my hospital bag for #1. It was her very own baby doll to take care of, and a book about being a big sister. We told her that her little brother got it for her. She loved it. Once we came home, we let her help as much as possible too, even if it was just getting diapers for me, or helping to burp him. I got her a mini ergo, and there were many days she would follow me around with her baby doll in the ergo, while I was wearing #2. Or she would burp/nurse/change her baby as I was doing it.
Theyre kids. Theyre resilient. And at the end of the day, having a sibling isnt going to ruin him for life. Just take some extra time now to help prepare him and make sure he knows that you understand that hes upset, but you think he'll be a great big brother.