Republicans in Northern Virginia yesteday took three of the region's 27 state legislative seats away from Democrats.

The GOP made its strongest gains in northern Fairfax County, where Republicans gained two members in the House of Delegates. By filling a vacant seat and defeating one-term incumbent Democrat Kenneth R. Plum.

"I think that this victory is indicative of a new wave of Republicans in Fairfax," said 26-year-old John S. Buckley, an education consultant whose victory makes him one of the youngest lawmakers in the 140-member state legislature.

Buckley, who defeated Plum, was joined in victory in the northern portion of the county by Republican John H. Rust Jr., an attorney.

In southern Fairfax, a Republican loss for conservative Del. Robert L. Thoburn was offset by the victories of James H. Dillard, a former delegate, and Lawrence D. Pratt an executive and lobbyist fr the Gun Owners of America.

Republicans in the region now hold 10 seats in the House and Senate to the Democrats' 17.

But Republicans failed yesterday in two key Senate races. Senate Majority Leader Adelard L. Brault soundly defeated his 22-year-old Republican challenger, John M. Thoburn, winning nearly 55 percent of the vote.

Brault, a 14-year veteran of the Senate, last night denounced his conservative challenger, saying that Thoburn in the last days of the campaign distributed flyers filled with "misrepresentations and lies."

Brault, who raised more that $32,000 for his campaign, said that had he not had "substantial resources in money and friends, I might well have been defeated by these tactics."

The Fairfax Fair Campaign Practices Commission found on Monday that Thoburn misrepresented Brault's voting record by claiming the senator voted to increase the sales tax on food and was pro-abortion.

"My integrity has never been questioned before; I've never been called a liar before," Brault said. "To have this done by a 22-year-old college dropout is a little difficult to take."

Thoburn, who is on leave from his college studies in economics, ran a strong campaign with the support of the same conservative Republican forces that failed to re-elect his father, Del. Thoburn.

In the race for the only vacant Senate seat in Northern Virginia, Democratic Del. Richard L. Saslaw defeated former Republican Del. James R. Tate with 53 percent of the vote.

Saslaw takes over the Fairfax County seat vacated by Omer L. Hirst, longtime Democratic dean of the area's Richmond delegation.

Tate, who outspent Saslaw three to one and raised more money ($37,569) than any other legislative candidate, had been expected to run close to his Democratic opponent.

But apparently Saslaw's door-to-door campaigning, in which he claimed to have contacted more than 15,000 registered voters since the June primary, overcame Tate's financial strength.

In rapidly growing Prince William and Loudoun counties, three Democratic delegates known as "the three B's" overcame the challenge of three Republicans who called themselves the "leadership team."

Incumbent delegates Earle Bell, a Loudoun businessman, Floyd Bagley, a Prince William attorney, and David Brickley, vice president of an investment company, soundly defeated thre Republicans who raised more than $40,000 between them in their unsuccessful challenge.

Elsewhere in Northern Virginia legislative races there were no major surprises. Democratic incumbent senators Clive L. DuVal Charles L. Waddell, Joseph V. Gartlan Jr. and Edward M. Holland all picked up easy victories.

Across the state, there were two Republican upsets. Veteran Sen. William B. Hopkins of Roanoke was defeated by former Del. Ray L. Garland, a smooth public speaker who had argued that Hopkins ws not qualified to manage the State's business.

Also defeated was Sen. Coleman B. Yeatts of Chatham, a seven-year veteran of the Senate. He was trounced by Danville Republican W. Onico Barker, a funeral director.

In northern Fairfax County, Del. Plum was the one Democratic incumbent knocked out of the five-member delegation from the 18th House District. The district now will be represented by four Republicans and one Democrat.

"The Republicans fielded a strong slate and they organized well," said Dean E. Brundage, one of the losing Democrats in the 18th District. "In the last couple years there has been a trend to Republicanism in the suburbs."

That trend, however, did not bode well for the conservative father-son Thoburn team in southern Fairfax.

They had targeted for defeat by Northern Virginia members of the National Organization for Women and the National Women's Political Caucus.

Kathy Wilson, a caucus spokeswoman, said yesterday the senior Thoburn's loss in southern Fairfax was a mixed blessing because he was replaced by the equally conservative Pratt, also an opponent of the Equal Rights Amendment.

The Thoburns last night said they were not discouraged with their losses. John Thoburn said his ability to garner 45 percent of the vote gave Brault "the biggest political scare of his life."

Robert Thoburn, a fundamentalist minister, said he plans to challenge Democratic Rep. Herbert E. Harris II next fall.