Fahd al-Ariny, held for more than three months, and Shaim al-Hamzani and Mohammed al-Mayouf, both held for six months, left detention on Monday night, Khaled al-Omair told Reuters.
"They were not questioned or charged at all. They were just put in prison and now let go," he said, describing the three men as Islamist reformers.
Ariny was one of the organisers of a petition launched in February calling for an elected parliament and a constitution based on Islamic principles.
Three other men in a separate group of 10 arrested in February were also signatories to the petition. The group remains in detention on suspicion of funding Islamist militants.
The petition accused the government of preventing reformers from travelling abroad, closing down Internet sites, banning public demonstrations and threatening state employees with dismissal for expressing opinions contrary to government policy.
There has been no official response to the document.
Saudi Arabia, the world’s biggest oil producer and a long-time U.S. ally, is an absolute monarchy with an unelected parliament and no political parties.
