Iraq is witnessing its deadliest violence since 2008, when it was emerging from a prolonged and bloody sectarian conflict between Sunni and Shiite Muslims.
The woman and her two adult daughters were killed in their home near Baquba, north of the Iraqi capital.
A Sahwa anti-Al-Qaeda militiaman said the women may have been informants for the security forces.
Gunmen also killed two brothers who were former Sahwa members near Baquba, where a bomb in the city killed one person and wounded five.
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In Tikrit, also north of Baghdad, a roadside bombing killed a man and his 11-year-old son as they walked in the city centre.
Gunmen also killed a judge's two bodyguards in the northern province of Nineveh, and militants attacked a police checkpoint with automatic weapons, killing a policeman.
The judge was not with the guards when they were killed.
An explosion near a market in Baghdad and another on a main road killed three people and wounded 10.
Security forces are also often targeted.
Violence has increased markedly this year, especially since an April 23 security operation at a Sunni anti-government protest site that sparked clashes in which dozens died.
Protests that erupted in Sunni-majority areas in late 2012 are ongoing amid widespread discontent among Sunnis who accuse the Shiite-led government of marginalising them.
Experts say Sunni anger is the main cause of the spike in violence this year.