Independent counsel Lawrence E. Walsh yesterday petitioned the U.S. Court of Appeals for the District of Columbia Circuit to reconsider a July decision by a three-judge appellate panel that set aside the three Iran-contra criminal convictions of former White House aide Oliver L. North.

In a 2 to 1 decision last July, the panel ordered elaborate hearings to determine if North's entire trial had been impermissibly tainted by earlier testimony he gave Congress under a grant of immunity from prosecution.

The ruling, if it stands, would be a sharp setback to Walsh's Iran-contra prosecutions, requiring extensive post-trial proceedings and possibly a new trial or even dismissal of charges against North. It could also affect Walsh's other major conviction, that of John M. Poindexter, former national security adviser to then-President Ronald Reagan, who was found guilty of five Iran-contra crimes at a trial earlier this year. Like North, Poindexter had testified to Congress in 1987 under a grant of immunity.

To discover whether the North prosecutors used improper evidence, the appeals court judges had ordered a "witness-by-witness {and} if necessary . . . line-by-line and item-by-item" examination of grand jury and trial witnesses to determine if any of North's immune testimony influenced their statements.

Walsh in his petition yesterday said the court's July decision "represents an unprecedented and unwarranted limitation" on a Supreme Court decision that set up rules for a U.S. district judge to determine if the government was misusing immune testimony.

Walsh adopted the language of Chief Judge Patricia M. Wald, who in dissenting with the majority wrote that the decision makes "virtually impossible" the criminal prosecution of anyone who has testified publicly under a grant of immunity.

Walsh asked that the rehearing take place either before the original three-judge panel or before all the appeals court judges meeting en banc. He could have gone directly to the Supreme Court with his petition, but chose the more cautious approach of asking the appellate court to reconsider its decision.

In a related matter, the Iran-contra trial of Thomas G. Clines, a former CIA agent who participated in the North-directed secret network supplying arms to the contras, began yesterday in Baltimore with the selection of jurors.

Clines was indicted last December on four counts of violating income tax laws. Specifically he was accused of failing to report on his federal tax forms the full amount of money he received for his role in the Iran and contra arms sales and not disclosing his interest in foreign financial accounts as required by law.

Clines worked in the "facilitation, acquisition, and delivery of arms," according to government court papers. He was paid "a percentage of the arms sale profits," which in 1985 and 1986 totaled "approximately $875,000," the government said.

Clines reported on his 1985 federal tax returns a gross income from business activities of $265,000. The government in court papers said it "expects the evidence to show that during 1985, approximately $423,000" went to Clines from the organization directed by North and retired Air Force major general Richard V. Secord. The organization was termed "the enterprise," according to congressional investigations. That year, Secord and Clines through the enterprise made money selling arms to the contras.

On his 1986 federal tax returns, Clines reported his gross receipts as a businessman were $402,513. The government has claimed he received $459,000 just from the enterprise's Iran-contra activities, which included selling arms both to the contras and to Iran.

During the 1987 congressional Iran-contra hearings, an accounting put together by the House-Senate investigating committees from materials supplied by Albert Hakim, the financial manager of the enterprise, showed Clines received a total of $1,009,903 in the two-year period.

Secord is expected to testify today, the first time he has appeared as a government witness in one of Walsh's prosecutions.