Lawyer Abd al-Rahman Al-Lahim said Abd Allah al-Hamid, Matruk al-Falah and Ali al-Dumaini were told around midnight that the closed session would be held the next morning.
He said the defence team were also given no advance notice.
The three men had previously refused to participate in their hearings unless they were open to the public.
"They refused to attend the court today," Al-Lahim said. He said no date had been agreed for another hearing in the trial, which has put Saudi Arabia's promised reforms in the spotlight.
Pressure to reform
The absolute monarchy, under pressure from inside and abroad to grant some popular political participation, has said it will hold municipal polls for the first time in decades next year.
But it arrested more than a dozen reformers six months ago in a move which drew criticism from its close ally the United States. Most were released but the three men remain in jail.
Saad al-Faqih of the London-based Movement for Islamic Reform in Arabia told Aljazeera.net the reformers were arrested after they refused to apologise for presenting a petition to the royal family asking for a constitutional monarchy.
"Interior Minister Prince Nayif told them in blunt terms that there was no room for a petition like this," he said.
Prince Nayif rebuked the
reformers for the petition
"Their case was adopted by human rights groups and the US State Department and that was embarrassing for the regime," Faqih said.
The charges against them include causing instability and collecting signatures for political petitions. It was not clear what sentences the charges carry.
The trial was adjourned in August after supporters surged into the courtroom shouting "reform, reform".
Hamid has been jailed several times by Saudi authorities.
Falah has taken part in drafting reform petitions while Dumaini is a liberal author and poet.