Ali al-Dimaini, Abd Allah al-Hamid and Matruk al-Falih, who have been held for more than a year, "were informed Tuesday that the hearing will take place Saturday morning", said lawyer Khalid al-Mutairy.
He said the hearing will be held in camera (in chambers) as in earlier hearings.
The judge had adjourned the hearing for three weeks on 12 March after a request by the prosecutor for additional time to gather more evidence against the defendants.
Mutairy said he did not expect a verdict to be reached in Saturday's session, which he thought would focus on examining the prosecutor's new evidence.
Al-Dimaini, al-Hamid and al-Falih were arrested in March last year on charges of demanding a constitutional monarchy in the oil-rich kingdom.
The three, whose trial opened in August, are also accused of "using Western terminology" in demanding political reforms. They also allegedly questioned the king's role as head of the judiciary.
The defendants were among more than 10 activists arrested at the time. The others were released in the same month after pledging to stop lobbying publicly for reform.