M.P.
First, I would suggest using the word "gentle". Such as, be gentle with your brother. He is very little right now, and he can get hurt very easily. So please be gentle with him."
Second, my daughter is now 12 and my son is 5. They fight, and roughouse everyday. No matter how much we tell them to stop, they still do it. It is just what siblings do, I guess. This is what i have been told.
As in the age right now, just make sure he is not hurting the younger one. Make sure you emphasize how it makes the baby feel. How would he feel if someone did that to him? He will soon understand, and may stop. If the 11mo. old is laughing or smiling at the 3 year old, the yonger one may think it's ok.
About the sharing, he will not want to share anything. But to help promote sharing, have him put away his most precious things to not share, and have him share something he doesn't really care for. That would be a start.
Good luck!
M.