The main similarity, to my mind, is that all three were regime changing situations: totally conquered nations with a managed transition to another system of government.
Japan and Germany were managed well - strangely by keeping the existing admininstrative structures somewhat intact (at least in Germany to some extent - I'm not so sure about Japan). While Iraq was a total failure (or rather, hardly a complete success) .
However, Japan and Germany were heavily occupied for decades after WW2, so it may just be a matter of time before Iraq goes the same way.
Additionally, Japan and Germany were relatively modern industrial economies, while Iraq was, and is, relatively undeveloped (?) (though oil-rich) and still, essentially, tribal(?).
But, this is only my (admittedly ill-informed) opinion.