As the power vacuum created by US withdrawal is quickly filled by the Shia we’ll see a down turn in sectarian violence not the ethnic cleansing many fear. Once they have political control of Iraq what do the Shiites stand to gain through annihilatory tactics waged against the Sunni minority? Such actions would certainly provoke Jordan and Syria as the refugee flood becomes a tidal wave. The Sauds are already waving their fists in response to Sunni deaths at the hands of Shiites and their perceived threat of Iran’s growing influence.
Actually I don’t see the Shia filling the power vacuum created when the US leaves. The Iraqi Sunni are quite adept at wreaking havoc and I think that would increase with the US gone. The threat we pose is political, whereas the Shia threat is existential.
I also think I misused the word “state” inaccurately. Most likely the three areas would be a Shia state in the South, a Kurdish one in the north, and a wild, violent region in the middle. I don’t see the Shia (large, unorganized, and ununited) being able to impose a monopoly of violence against a more organized and much more united (smaller in size) Sunni region. Especially if the Saudis and AQ are able to make spoiling attacks and fund the warring factions.