Just because something is called a "Militia" does not make it a "Militia". For example the Police in Moscow real name is "Militia" not Police. This is do to the fact that Under Communism the name "Police" was tied in with the Czarist use of Police to suppress the Working Class, with the Revolution the term "Police" was no longer acceptable so when Moscow (and other Soviet Cities) reformed their Police they called them "Militia" for the term "People's Militia" sounded better to them than "Police".
Ramadi is a nice size City, with a population of around 400,000 people. Assuming "Normal" population distribution (50% of the population is Female, and roughly another 25% of the population is under 10 or over 45 or disabled), a Militia for Ramadi should be about 100,000 men NOT 3000 men (Iraq has a very young population compared to Western Countries, so the above number may be Conservative). Thus this force is NOT a Militia but something else.
If it is NOT a Militia what is it? That depends on who is in the unit. If these are Shiites and/or Kurds this may be an attempt to "Colonize" (In the Roman sense of that World) Ramadi with more loyal population (Saddam did this in regard to Mosel, as did other rulers throughout history). As a Roman type Colony, it may purpose is to provide a local presence of the Occupying power AND to provide a local force Loyal to Occupying power NOT the people the Colony is settled among (A recent example are the Jewish Settlements on the West Banks and Gaza).
It can be like the Moscow Militia, full time Police but given a nicer spin in their name than "Police".
It may even be a full-time military unit given permanent homes to provide permanent overlordship over the subject population (Many Roman Colonies started as such Full Time military units).
Now do NOT confuse 19th Century Colonialism with the above Colonialism. Most 19th Century Colonialism was to take over a country, put someone friendly in charge but NOT to sent people to settle among hostile forces (The biggest exception I know of were the French in Algeria and the Russians into what is now the Baltics, the Ukraine and the former Soviet Republics of Central Asia).
Colonialism in America is also different, the early Colonist were NOT soldiers settled among hostile but people living among basically vacate land, vacated by Small Pox and other diseases introduced since Columbus's voyage in 1492 (You would NOT have substantial settlement of ex-soldiers in the West till the French and Indian War of 1754-1763 and the American Revolution 1775-1783 and even these settlements tended to be more land as payment of wages than to maintain a Military force among the natives).
Thus the best explanation may just be these 3000 men are to be a new Roman type Colony (If Kurds or Shiites) or a local "leadership" if Sunni. Machiavelli recommend such colonies among recently conquered territories, it was a quick way to pay off your troops (With the land taken during the War) AND give them great incentive to stay or lose their new homes. Machiavelli recommended it for it works PROVIDED you are NOT driven out or the Colonists are NOT driven Out.
More about the City of Ramadi:
http://en.wikipedia.org/wiki/Ramadi