Sibling rivalry?

I am at my wits end with my kids. Ages 9, 6, and 4. They don’t get along. Any combination of two can play together amicably provided one is a leader and the other a follower , but if all three are left alone for any length of time someone ends up crying. They rarely have a kind word to say to or about each other. They don’t help each other until instructed to by a grown up. They don’t stick up for each other. They see each other as competition but never as teammates. They delight in finding wrong in one another but when asked to say something nice about a sibling they all go blank. I have siblings but I didn’t grow up with them and my husband is an only child. Is this just normal sibling behavior and we don’t understand it or should I be doing something? If there is something I can do for them, what?? via /r/Parenting http://bit.ly/2M9x9us