A three-judge court announced today that it had reached a verdict on the Benigno Aquino murder case and will make it public Nov. 20.

Some lawyers described the announcement as an attempt to preempt Supreme Court action on a petition to declare the trial null and void.

The clerk of the court trying 26 men for the Aug. 21, 1983, killing of opposition leader Aquino said the defendants and their lawyers had been told to be present in court for the verdict.

On trial are armed forces chief of staff Gen. Fabian Ver, 23 other military men and a civilian. President Ferdinand Marcos has said that if Ver, a close friend, is acquitted, he will be reinstated as chief of staff. He has been on leave. The Reagan administration has warned Marcos that reinstatement of Ver would have serious repercussions in Washington.

Augusto Amores, one of the three judges of the court, which usually tries corruption cases against public employes, told reporters that the decision was unanimous.

The move came a day after 31 prominent Filipinos petitioned the Supreme Court to declare a mistrial to stop what they said would be a whitewash by the trial court.

One of the lawyers involved in the petition, Arturo de Castro, called today's announcement "an improper and imprudent act." He said the question of whether the court was qualified to sit must be resolved first. He said he would file another motion to urge the Supreme Court to issue a restraining order immediately.

The seven-month trial, which ended in late September, saw the gradual erosion of the prosecution's case against the accused, especially Ver.

Ver is accused as an accessory of covering up a military conspiracy to kill Aquino, who was shot at Manila airport as he was arriving home to resume political activity. The defendants also are charged in connection with the killing of Rolando Galman, who was shot immediately after Aquino's death by soldiers who said he was the killer.

The petition for a mistrial said the prosecution was under tremendous pressure and the evidence presented was incomplete. It also said Marcos had made improper statements during the trial favoring the defendants and had cast doubts on the finding of an official inquiry that had indicted all 26 men.

Among those who signed the petition were three former justices of the Supreme Court, a retired armed forces chief of staff, several human rights lawyers, businessmen, priests and nuns.