In the deadliest attack, gunmen broke into the house on Baghdad's western outskirts of an anti-Al-Qaeda militiaman in the early morning, killing him, his mother and his brother, medical officials said.
The fighter was a member of the Sahwa, a group of Sunni tribal militias that sided with the US military against Al-Qaeda from late-2006, helping to turn the tide of Iraq's insurgency.
Also on Wednesday, bombings in the northern cities of Kirkuk and Mosul left a policeman dead and nine people wounded, officials said.
The apparent rise in violence, which left 271 Iraqis dead last month, the highest such figure since August 2012 according to an AFP tally, comes ahead of provincial elections due on April 20.
The polls will be the first in Iraq since 2010, and come after the withdrawal of US forces at the end of 2011.