I have DS (5) he played up a bit when X left. I can't imagine how tough it could be with three, and 7, 6 and 2? Harsh!
Can you try talking to them, separately? see what they are thinking and tell them that you are sad, scared and need their help. Tell them that the Team needs them to help, not fight, and that no matter what they think, feel or worry about, you are there to listen and to help them in every way. OK so that would work with the 2 eldest, but getting them calm will help the 2yo.
What are you telling them about their Dad? if nothing at all, they may be confused. You may have to come up with something to tell them, in an age appropriate way.
Reward charts? would they work? could you try?
It will get better, in time, keep talking to them, keep listening and observing. You have all been through a traumatic situation, it's only natural that there will be some fall out.