The killings occurred on Tuesday in the Karrada district of central Baghdad.
They came close on the heels of an attack on a minibus in Baquba that killed 13 Iraqi workers travelling to a US base.
In the Baghdad attack, a Reuters television cameraman at the city's St Raphael's Hospital saw three bodies being pulled out of a dark blue car that had been sprayed with bullets. Their wounded driver was loaded into the back of a truck.
Police sources named the victims as Sheikh Mujbil al Sheikh Isa, Aziz Ibrahim and Dhamin Hussein Ileywi.
The 71-strong committee is due to deliver Iraq's new constitution by 15 August.
Fifteen Sunni members joined it last month, making it the first nationwide political body to include significant Sunni representation since the new government took power in April.
Fifteen Sunni members joined
the constitution panel last month
The other members are mostly Shia and Kurds, elected to parliament in a January vote when most Sunnis stayed home.
Two members of parliament have been assassinated since the elections, but Tuesday's victims were the first members of the constitution-drafting committee to be killed.
Earlier on Tuesday, armed men in two cars attacked a minibus taking workers to a US base in Iraq, killing 13 people.
Aljazeera reported 10 of the dead men worked at the US military base at al-Faris airport.
The men were killed in Baquba, just north of the capital, after the attackers blocked the bus and opened fire.
Three other civilians died when the bus careered into their car, a Baquba police source said.
Bodies had been taken to two hospitals in Baquba and nearby Khalis.
The constitutional panel
members were killed in Baghdad
Armed men ambushed a police vehicle on Tuesday in northern Mosul, killing six people, and shot dead a neighbourhood official, police said.
The first attack targeted Sayyed Zanoun al-Naimi, a local official in the central neighbourhood of Jazaer, who was killed by armed men in front of his house, police Brigadier General Saeed Ahmed al-Jbouri said.
The second attack targeted a police vehicle that was transferring a prisoner from Mosul to the prison of Badoah, 20km to the northwest, a police official, Wathiq Mohammed, said.
Three officers, including a colonel and a major, as well as two police and the prisoner were killed in the shooting, he said.
Mohammed added that police forces sent to the area were fired at by an attacker, who wounded one of them before being killed.
Earlier in the day, US troops shot dead Amer Ziad, a local official of the Iraqi Islamic Party, the largest Sunni Muslim political group in the country, party member Noureddine Saeed al-Hayali said.
Al-Jbouri said the incident occurred in the afternoon when Ziad tried to overtake a US patrol that opened fire at him, fearing he was an attacker.
Also on Tuesday, two people were killed and four others wounded after an Iraqi police patrol was hit by a roadside bomb in the northern city of Kirkuk.
One of the dead was a policeman, while the other was a member of the Patriotic Union of Kurdistan (PUK), the political party of Iraqi president Jalal Talabani.
Police guard a damaged vehicle
after a roadside bomb attack in Kirkuk
"The IED (improvised explosive device) was targeted at a passing police patrol in which one policeman was killed and one wounded," police Colonel Adel Zaein al-Abidin said.
The incident occurred at the al-Wasiti neighbourhood near central Kirkuk at 8.30am (0430 GMT), he added.
"The other person who died was from the PUK," the colonel added. The three other wounded were civilians.
Iraq is reeling under a blitz of attacks since Friday in which scores have perished and wounded.
The deadliest attack came in the southern Iraqi town of al-Musayyib on Saturday evening when a bomber blew up a gas tanker, killing 83 people and wounding 151.
Meanwhile, doctors at al-Yarmouk hospital were staging a protest on Tuesday after a doctor was beaten and humiliated by the Iraqi National Guard, a doctor said.
The doctors called on the government to provide better protection so they could perform their duties.
Iraqi doctors want better
protection to do their jobs
Dr Riyadh Salman Mahdi, head of the general practitioners in al-Yarmouk hospital, told Aljazeera: "An Iraqi national guardsman attacked a doctor on Monday afternoon at the emergency section in al-Yarmouk hospital in Baghdad.
"This is the not the first such incident in the hospital," he added.
"We have staged a sit-in, calling on the Iraqi government to provide full protection to the doctors in the hospital, so they can perform the necessary medical service."