The three men supplied explosives to the perpetrators who carried out the Thursday bombing in Zahedan, 1,000 miles southeast of the capital Tehran, said a statement issued by the judiciary.
Ebrahim Hamidi, the head of the justice department in Zahedan, said that the men were actually arrested before the mosque bombing, but they had "confessed to importing explosives into Iran and providing them to the main person behind the attack."
He added that the men, identified as Haji Nouti Zehi, Gholam Rasoul Shahoo Zehi and Zabihollah Naroui, were also involved in several other bombings including a bus attack in 2006.
Jundallah or God's Brigade, a Sunni militant group believed to have links with al-Qaida, claimed responsibility for the attack.
The group is composed of Sunni Muslims from the Baluchi ethnic minority who have been fighting a low level insurgency in southeastern Iran for years, complaining of persecution by the overwhelmingly Shiite and Persian Iranian government.
The Baluch minority can be found in Iran, Afghanistan and Pakistan.
Iran has blamed the U.S. and Israel for trying to stoke sectarian tension with the Sunni Muslim minority. The U.S. condemned attack Friday.
In March 2006, gunmen dressed as security forces killed 21 people on a highway outside Zahedan in an attack authorities blamed on "rebels," though Jundallah was never specifically named.