I agree with Noz that dividing Iraq in three would not stop the fighting and would cause at least as many problems as it could solve, at least in the short term. However, this is no longer about just Iraq. It’s about a regional sectarian war that has ethnic implications as well. Insofar as any outside parties can be helpful in putting an end to the violence, it probably isn’t a good idea to try to put everything back together inside the Iraq and Syria nation-state boxes.
The problem, as I see it, is that getting consensus on new dividing lines would probably be just as difficult as trying to put Iraq and Syria back together. The advantage isn’t that it would be easier to do, but that the end result would have the promise for a more rational sorting of populations.
Either way, I don’t expect the fighting to stop for the foreseeable future. This kind of sectarian fighting coming on top of a fortune in fossil fuels in lands with countless shrines and holy places just isn’t likely to burn itself out until a generation lies in ruins.
That doesn’t mean that the international community should resign itself to this fate for the region. But it shouldn’t waste time and energy trying to put all these shards of eggshell back together again.