Alright let me put out a quick statement as to why separating them into states is the last thing we could possibly want.
Problem 1. Kurdistan.
One of the three states (the Northernmost) would be Kurd dominated, the problem is that there are also a large number of Kurds in neighbouring parts of Iran and Turkey, once one part of 'Kurdistan' comes into being you can bet anything there will be some form of rebellion from a portion of the Kurds in Iran and Turkey. Basically there would be civil wars in two countries, one of which is the only Arab country that actually really friendly towards the west in the Middle East.
Problem 2. Shia-Sunni tensions.
These two cultures are hardly friendly, splitting them into two religiously divided states, the Shia one of which would be heavily backed by neighbouring Iran, could potentially start a devastating war.
Basically the status quo, while pretty terrible, is going to have to be maintained. The US is going to have to stay in the mess its in until the Iraqi government can successfully control their own country. Leaving would be bad and the splitting the country is the worst possible case.
