A spokesman for the Iraqi Islamic Party, Noureddine Hayali, said that masked gunmen kidnapped the three men early Friday in a southern neighborhood in Mosul.
The three members were abducted while they were hanging posters urging people to vote in a referendum on the country's new constitution, scheduled for October 15.
The shootings came a day after an attack in Ramadi on Sunni leaders who had been discussing the constitution process.
The gunmen asked the attendants to end their meeting, then opened fire on them, said Omar Seri, secretary of the governor of Anbar province.
Three members of the Sunni Association of Muslim Scholars and a bodyguard were wounded in the attack, Seri said.
Iraqi leaders are still drafting the constitution.
One correspondent says Iraqi politicians hope they can reach an agreement on the draft constitution ahead of the extended deadline of 22 August.
She warned that the recent attacks could further delay the deal.
The draft constitution must be approved by the National Assembly before it can go to the nationwide referendum.
Meanwhile, a U.S. general said that U.S. occupation forces have killed or arrested 170 foreign fighters in northwestern Iraq.
Major General David Rodriguez, commander of U.S. forces in northwestern Iraq, said that about 70 fighters have been arrested in his area of operations over the past three months, and about 100 more have been killed in clashes with U.S. forces.
Other developments:
- Two Iraqis were killed by a roadside bomb near Tikrit, 180 kilometers north of the capital, police said
- A roadside bomb exploded near a U.S. military convoy in Baghdad, wounding one Iraqi civilian but causing no American casualties
- An Iraqi contractor who worked for the U.S. military was shot dead by gunmen near Kirkuk, police said
- Rebels killed a city council member in the northern city of Hawija