Their case had been referred to a single-judge court on 1 December, as the judicial panel that was presiding over the trial declared it lacked the jurisdiction to look into the case.
But the court of appeal annulled that decision, returning the case to the same judicial panel, meeting a demand by the prosecutor, the son of the reformists Amir al-Falih said on Tuesday.
He said the trial, which opened in Riyadh last August, would convene again in the same court.
Ali al-Dimaini, Abd Allah al-Hamid and Matruk al-Falih were arrested in March 2004 on charges of demanding a constitutional monarchy in the oil-rich conservative Muslim kingdom.
Elections to municipal councils
were held on 10 February
The last hearing was held behind closed doors and was preceded by the detention of seven protesters, including two brothers of al-Hamid, and two journalists, at the courthouse.
Two of those detained, including one of al-Hamid's brothers, are still behind bars. The trio's outspoken lawyer, Abd al-Rahman al-Lahim, was arrested in November and is yet to be officially charged.
The crackdown on democracy advocates has cast a shadow over a cautious process of reform in Saudi Arabia, where unprecedented polls to elect half the members of new municipal councils kicked off on 10 February.