WASHINGTON (AP) _ Legislation to weaken the Ethics In Government Act would unconstitutionally pardon former Rep. George Hansen and two other men who received criminal convictions under the law, the Justice Department said Wednesday.

The legislation would prohibit criminal prosecutions of members of Congress and top federal officials who violate the financial disclosure requirements of the 1978 law.

Language in the bill making the prohibition retroactive to 1978 would wipe out the convictions of Hansen, an Idaho Republican, and two other former federal officials who filed false disclosure statements, John Keeney, deputy assistant attorney general, told a Senate hearing.

''Such action would be unprecedented,'' Keeney told the Governmental Affairs subcommittee on oversight of government management hearing on the bill sponsored by Sens. Orrin Hatch, R-Utah, and Alan Cranston, D-Calif.

''We believe Congress has no authority to enact the retroactivity section of this bill and that, in addition, the retroactivity provision would be an unconstitutional intrusion on the president's constitutional power to pardon,'' Keeney said.

But in testimony that clearly disturbed subcommittee members, Keeney said the Justice Department had no position on the central question raised by the bill: Should officials face only civil fines for filing false statements about their finances.

Currently either civil or criminal charges can be brought against violators. The top civil penalty is a $5,000 fine. Maximum criminal penalties are five years in prison and a $250,000 fine.

Keeney said that because the Justice Department brings criminal prosecutions only in the most aggravated ethics cases, the retroactivity provision would affect only three people.

One, he said, is Hansen, who entered a federal minimum security prison last Friday to begin serving up to 15 months for his 1984 conviction for falsifying financial disclosure forms.

Keeney identified the others as former Senior Assistant Postmaster General Paul E. Jaquish and Herman E. Thomason, former deputy director for science and engineering at the National Aeronautics and Space Administration .

Keeney said only two other people have faced criminal charges under the ethics law - former Assistant Navy Secretary George A. Sawyer and Harry E. Claiborne, chief U.S. district judge in Nevada.

Both were acquitted of the charges, but Claiborne recently began serving a two-year sentence for income tax evasion and, because he refuses to resign, is the subject of the first congressional impeachment proceeding since that of former President Nixon.

Keeney said the question of outlawing criminal prosecution for future violators is one for Congress to decide without guidance from the Justice Department.

He said internal department debate over taking a position ranged over a number of positions, with some people arguing that criminal penalties are valid tools and others saying there should only be civil sanctions because of ''the intrusive nature'' forcing officials to reveal financial data.

''I cannot understand on such a major issue as the conduct of public officials why the Justice Department does not say yes or no,'' said Sen. Warren Rudman, R-N.H.

Sen. Carl Levin, D-Mich., added: ''This is some of the most discouraging testimony I've heard - ever. ... It's very disappointing the Department of Justice won't tell us what they think.''

Also staying officially neutral on the question of dropping criminal penalties was the Office of Government Ethics. But its director, David Martin, said he personally favors keeping criminal sanctions.

Martin said his original testimony supporting criminal penalties was rejected by the Office of Management and Budget, which routinely clears statements administration officials make to Congress.

Gary Davis, staff attorney for OGE, said it was his understanding that the decision for administration witnesses to take no position ''was coordinated with the Department of Justice and the counsel to the president.''

Neither Hatch nor Cranston - nor anyone else - testified in favor of the bill. But in a prepared statement, Hatch said that because the ethics law does not specifically allow criminal prosecutions, it was Congress' intent that they not be allowed.

The criminal cases to date have been brought under a catch-all law prohibiting fraud against the government. Ruling in the Hansen case, the U.S. Court of Appeals here said this practice is legal. The opinion was written by Judge Antonin Scalia, nominated by President Reagan last week to the Supreme Court.

Anne McBride of Common Cause, the self-described citizens lobby, told the subcommittee that the legislation is ''a frontal assault on the ability to enforce ethics laws.''

She said its enactment would create a double standard: one for lawmakers and high officials and another for average citizens, who would remain liable for criminal charges for making false statements on a variety of federal forms.

After the hearing, subcommittee chairman William Cohen, R-Maine, was asked whether he planned to take any action on the bill. ''I doubt it,'' he replied.