The blast was apparently targeting a police patrol in a commercial area in Balad Ruz, according to a senior police officer.
The violence a day before the fifth anniversary of the U.S.-led invasion was a grisly example of the dangers still facing Iraqis despite recent security gains with an influx of American troops.
If the attacker's identity is confirmed, it would raise to eight the number of women suicide bombers so far this year.
That has raised concerns that extremists are making greater use of women as suicide bombers because explosive belts are easier to conceal under their traditional flowing Islamic robes and they are often not treated with the same suspicion as men.
In the worst such attack, two women laden with explosives struck two pet markets in Baghdad on Feb. 1, killing nearly 100 people in one of the deadliest days in months.
On Monday, a female suicide bomber also blew herself up amid a group of Shiite worshippers near a mosque in the holy city of Karbala, killing at least 32 people and wounding 51.
The three killed in Wednesday's attack included a policeman, and 12 others were wounded, including three policemen, women and children, the officer said, speaking on condition of anonymity because he wasn't authorized to release the information.
Ahmed Salim, a 29-year-old policeman, said the explosion occurred shortly after his patrol arrived at the bus terminal.
"I didn't know what happened," he said from his hospital bed in the nearby city of Baqouba. "My face was cut by the glass of the vehicle and my leg was hit by the metal balls from the explosives belt."
Farther north, U.S. troops accidentally killed three Iraqi policemen and wounded another, the military said, the latest in a series of friendly fire incidents.
The Iraqi patrol, which was responding to an unrelated request for assistance, raised suspicion as it sped toward U.S. troops operating in a cordoned-off area, Spc. Megan Burmeister, a U.S. military spokeswoman, said in an e-mailed statement.
Coalition forces, perceiving a hostile threat, engaged the vehicle," she said, adding the incident is under investigation. "Coalition forces make every effort to engage only hostile threats and we sincerely regret when a tragic accident like this occurs."
Local police officials acknowledged the Iraqis had sent the patrol without informing the Americans and the troops apparently feared it was a car bomb.
As the war enters its sixth year, the number of attacks has dropped with the addition of some 30,000 extra American troops, a Sunni revolt against al Qaeda in Iraq and a cease-fire by radical Shiite cleric Muqtada al-Sadr's Mahdi Army militia.
But the levels of violence have been slower to decline in northern Iraq as militants have fled security crackdowns in Baghdad and surrounding areas.
A suicide car bomb also struck an Iraqi army building in the northwestern city of Mosul, wounding 14 people, police said.
In other violence, a bomb stuck to a taxi exploded in central Baghdad, killing the driver and wounding a passenger and three pedestrians, police said.
In other developments: