Is this a stage or should I be worried?

Hi ladies hope you can help me out, my son has just turned 3 in November he is normally and lovely friendly little boy who enjoys playing, With his cars and singing nursery rhymes. As he was the first grand child from both sides of the family he’s use to geting extra attention, and extra gifts. However he’s recently got into a bad habit of not wanting to share his toys with other children when they come over, and will sometimes even try to hit the other child with the toy if he feels they will try to take it. I try to correct his behaviour by removing him from the situation and telling him he can not play with other children if he is not nice to them on some occasions I have taken him out of the environment completely and told him we have got to leave now and go home because u are hurting ur friends. I’m really hoping this is just a learning curve due to the fact he’s been on his own for so long but does/has anyone else ever faced this issue, was it just a phase or should I be worried and any tips on how I can help him to change his reactions. He is due to start nursery next year and I’m worried. Hope someone can help out x