Ahmed al-Juburi, the former governor of Iraq's Salaheddin province, was sanctioned for corruption and has also "been known to protect his personal interests by accommodating Iran-backed proxies," Treasury said in a statement.
The two militia leaders, Rayan al-Kildani and Waad Qado -- the former a Christian and the latter a member of the Shabak minority -- were both sanctioned over "serious human rights abuse" by them or their organizations.
Treasury cited a video circulated in May 2018 in which Kildani "cut off the ear of a handcuffed detainee," and said that his forces have "systematically looted homes" and "reportedly illegally seized and sold agricultural land."
Qado's militia has meanwhile "extracted money from the population around Bartalla, in the Nineveh Plain, through extortion, illegal arrests, and kidnappings" and has "frequently detained people without warrants, or with fraudulent warrants."
Vice President Mike Pence said Thursday that the United States was imposing sanctions on the leaders of two Iranian-linked militia groups in Iraq, but the Treasury statement made no mention of the commanders being tied to Iran.
"Let me be clear, the United States will not stand idly while Iranian-backed militias spread terror," Pence told a high-level meeting on religious freedom, without naming the individuals targeted.
- Endemic corruption -
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The fourth person sanctioned is Nawfel Akoub, the former governor of Iraq's Nineveh province.
He was sanctioned for "corruption, including the misappropriation of state assets, the expropriation of private assets for personal gain, corruption related to government contracts or the extraction of natural resources, or bribery," the Treasury Department said.
Reached by AFP, Akoub denied the allegations and slammed Treasury's decision.
"I call on the Treasury to review its decision. They should check the information that they received more closely. This decision is invalid, politicized and based on false information," Akoub said.
Parliament voted to sack Akoub after an overloaded riverboat capsized in March, leaving 100 people dead, most of them women and children.
Iraq's anti-corruption Integrity Commission said earlier this year that officials "close to Akoub" had embezzled a total of $64 million in public funds.
The money, which was allegedly stolen in the aftermath of the ferry sinking, included nearly $40 million set aside to rebuild Mosul, which was ravaged by more than two years of Islamic State group rule followed by months of fierce fighting to retake the city from the jihadists.
Graft is endemic across Iraq, which ranks among the world's worst offenders in Transparency International's annual Corruption Perceptions Index.
Since 2004, a year after the US-led invasion that toppled Saddam Hussein, a total of $228 billion has vanished into the pockets of shady politicians and businessmen, parliament said last year.