S.H.
you sit on the floor between them. The toys are "yours" & "you choose to share with everyone". Take the time to divvy them up between the three of you....& simply role-play (without the boys being aware) the behavior you wish for them.
It truly can be as simple as that.....you are the leader & you teach them your life rules! Socialization & cooperation are the most important life skills to be taught to this age group....makes KG all the easier!