The three candidates seeking the Democratic Party's nomination to challenge Northern Virginia Congressman Frank Wolf used their first and perhaps only joint appearance last night to concentrate their attack on Ronald Reagan instead of each other.

The three candidates criticized Reagan's economic and defense policies and said they believe 1982 will be "a Democratic year." Former Virginia delegate Ira Lechner, the best known candidate, said Reagan administration officials are "radicals who are bent on destroying and tearing down the basic fabric of American society.

"The very last place that I would look to cut would be Social Security," Lechner told about 35 Democrats in the Loudoun County Courthouse in Leesburg. "My mother lives on Social Security."

Lechner and two former federal employes, Ted McLaughlin and Rose Thorman, are running in a June 8 primary in the 10th District, which includes Arlington, northern Fairfax County and Loudoun County. Republican Wolf, a former babyfood lobbyist, won the seat during his third try against incumbent Joseph Fisher, an Arlington Democrat, in the 1980 Reagan sweep.

Fisher had been expected to run again until Gov. Charles S. Robb appointed him to the state Cabinet earlier this year.

All three candidates last night criticized the federal budget deficit, which they blamed on the Reagan administration. Thorman sought to distinguish herself from her opponents by saying that 1982 will be not only a Democratic year but also "a year for the woman."

"Northern Virginia has never had a woman in Congress, and maybe it's the time," she said.

Thorman, an Arlington resident and former Interior Department employe, also said that Reagan's layoffs of federal employes was "the most vicious destruction of human beings that this country has ever seen."

McLaughlin, the only Fairfax resident of the three, said his chief concerns in the district are "the terribly low morale of the federal civil service" and the infusion of out-of-state money into Wolf's campaign.

Without naming Lechner, McLaughlin also criticized his Democratic opponent for accepting union money from outside the state. "It's hard for me to see the difference between a Republican pipeline that's clogged with special interest money from the outside, and a Democratic pipeline," he said.

Lechner and McLaughlin both congratulated their Loudoun audience for electing a Board of Supervisors that voted last month to support a nuclear weapons freeze.