The three were seized Saturday after they were found without passports while riding with a Kurdish driver, said Maj. Gen. Wathiq al-Hamdani, the police commander for Ninevah province.
He said they claimed they had authorization to travel in the adjacent Kurdish-controlled region but mistakenly entered Mosul, 225 miles northwest of Baghdad. Al-Hamdani said the three were under investigation.
The Iraq Foreign Ministry said it had no immediate information on the incident, which came just over a week after six Iranians were detained in a U.S. raid on a liaison office in Kurdish-controlled Irbil, although one was later released.
The U.S. has linked those still in custody to an Iranian Revolutionary Guard faction that funds and arms insurgents and militias in Iraq.
Iraqi officials have called for their release from U.S. custody as it faces a delicate balance in trying to secure Baghdad with the help of American forces while maintaining ties with its neighbors, including U.S. rivals Iran and Syria.