Oliver North, the former White House aide once praised by President Ronald Reagan as "a true American hero," was convicted Thursday of three felony crimes arising from the Iran-Contra scandal. He was found not guilty of nine other counts.

The jurors announced the verdicts on their 12th day of deliberation.North, a Marine lieutenant colonel detailed to the National Security Council, helped direct the Reagan administration's secret two-year effort to help the rebels fighting Nicaragua's leftist government after Congress banned official U.S. aid.

He was involved, too, in arrangements under which the United States secretly sold arms to Iran and also helped divert some profits from those sales to help the Contras.

Public disclosure of the affair in November 1986 began the worst crisis of President Reagan's eight-year presidency, a public furor that didn't subside until after televised congressional hearings that made North a national figure.

The maximum possible sentences for the convictions total 10 years in prison and $750,000 in fines.

North's contention, both in the congressional hearings in 1987 and during his three-month trial, had been that he had authorization for everything he did: from two successive bosses, National Security Advisers Robert F. McFarlane and John M. Poindexter, and "concurrence" from the late CIA Director William C. Casey.

He said he assumed that Reagan - who spoke out often in support of the Contra cause - also knew of his efforts and approved them. Reagan told the investigative Tower commission two years ago that he did not know of his NSC staff's efforts to help the Contras.

Reagan fired North the day the affair became public but telephoned him the same day and told him he was a hero.

The jury in North's trial was instructed by Gesell that "neither the president nor any of the defendant's superiors had the legal authority to order anyone to violate the law."

North was his own chief witness, spending six days on the stand _ much of it under withering cross examination. He testified that in November 1986, with the Iran-Contra affair about to be revealed, he shredded and altered documents on a list McFarlane had prepared.

"I had been led to believe that everything I was doing was done at the direction of the president," North said.

"You had been told by the president of the United States to destroy documents?" asked prosecutor John Keker. "Explain how you had come to that conclusion."

Said North: "Because everything I had done that was described in the documents on this list I had been told was at the direction of the president."