How do I get my 3 year old son to share?

I have 2 boys, one is 3 and the other is 4 months. I’ve tried making my eldest understand the concept of sharing, I’ve shown him through role playing and make him see it’s important to share. As he goes Nursery I’ve told him to share with other children as it’s not nice if he tries to snatch.

He doesn’t like it when I give rattles or soft toys to his baby brother and will take it away 🤷🏻‍♀️ Any advice? X