Siblings not sharing toys and not accepting a No answer

I have three kids: 9, 6, and 2 and a half years old. They give each other too many “No answers” when sharing toys. How do I explain the value of sharing and agree on when a “No answer” is appropriate? I would like them to be more open about sharing and more inclusive of each other.
Could you please talk about some good family games that will include all three kids?
Thank you very much!