How to help my toddler with his bad behavior's?

My son is now 3 years old and I have always felt he has some sort of behavioral/attitude problem. In school, he gets frustrated easily and doesn't seem to want to tell what the problem is, all he does is screams or tattle tales to the teacher about what other kids do, if they take his toy, have pushed him or did anything to make him cry. But for the most part the teachers tell me he listens and is a smart and good boy, when he is in a godo mood. At home then, he constantly wants his daddy's attention and just wants him to play with my son all day long. with the other baby boy we have who is 5 months, my older son won't allow my husband to hold the baby. then my son whines and screams for attention in different ways unless he gets it. then when we go to parties and have other kids there, my son will not share toys, he takes other kids toys and then plays with them himself and does not want to share...he also likes to play by himself, not with other kids, he only maybe 2 out of 10 times will sit with all other kids and eat, play, and or do what they are doing. what is this, and how to help him? My hub and I talk to him, we yell sometimes when it's out of control, but many times i feel like my son is very tantrum-possessive, clingy to daddy, and, does not want to play with other kids and wants to be a bad boy when we are outside with him...I am so scared now what to do? will my son ever get better?