In a perfect world, our children, regardless of their ages, would play together with never a cross or mean word. And of course, that rarely happens, but we can't help wanting them to get along happily. The reality is that children (and even mature adults) don't always get along happily.
Each child is unique, and you obviously have two who get along well and one who doesn't. Be grateful for the two who get along well together. Sometimes our children's personalities just do not mix well when they come too close. Some of this conflict will smooth out over time, often because we become involved in our separate interests, and we learn to be respectful of others.
I would not focus on the children playing together. Focus on respectful behavior toward each other. That means no name calling and a polite "No, thank you" if one does not want to join in the games of the others. Naturally, the younger ones will not be as mature as the older one. However, eventually they will catch up in maturity. Please emphasize to the older child that name calling and a lack of respect are immature and childish behaviors.
It might be helpful to have some family time when the whole family plays or works together. Your sons all need to understand how you and your husband get along with children, which basically means them. Some of my fondest memories are of family games of volleyball and basketball where my parents and older brothers and sister worked with us younger ones, teaching and encouraging us and allowing us to make mistakes without teasing us. We invented our own games too, so that all five children could join in with easier rules for myself and my younger sister. We could then participate more or less evenly with the older three. We often worked together at chores as a family with appropriate jobs for each age level. Name calling resulted in immediate punishment that was memorable enough for us to hold our tongues the next time we were tempted to tease or belittle each other.
If you read the lives of the saints, you might chuckle to discover that even some of the saints didn't get along too well with others.
God bless you and your children,