Three Republicans are seeking their party's nomination in the primary election Tuesday to run against Democratic Rep. Herbert E. Harris II in Northern Virginia's 8th Congressional District.

The candidates are John F. Herrity, chairman of the Fairfax County Board of Supervisors, and State Dels, Robert E. Harris and Robert L. Thoburn, both of Fairfax County. Polls will be open from 6 a.m. to 7 p.m.

A light voter turnout is expected in the district, which includes southern Fairfax County, Alexandria, Manassas, Manassas Park, Prince William County and northern Stafford County. Although it is a Republican primary, all registered voters in the 8th District are eligible to vote in the primary.

"We're not looking for a whole lot of voters," said a Prince William County election official.

Millard C. Rappleyea, secretary to the Fairfax County Electral Board, said he is expecting an 8 per cent voter turnout for the Republican primary. There are 114,000 registered voters in the southern portion of the county.

The lack of voter interst has made the candidates search much harder for votes.

Herrity has sent out a steady stream of press releases and has been making a number of phones calls and house calls to voters.

Harris, a third-term delegate to the General Assembly, has tried to focus on the people who normally vote in the primary, a tecnique he calls the "voter ID system."

Thoburn, a first-term General Assembly delegate and an ordained minister, has run his campaign out of the school he owns and operates. Much of the time, he says, is spent organizing and training campaign volunteers.

The three Republicans have focused their attention on the November opponent two-term incumbent Harris.

Herrity, who runs an insurance business out of his Springfield home, has been chairman of the County Board of Supervisors since 1975. Prior to becoming board chairman, he served as the Springfield area representative on the board for four years.

Robert Harris, an energy an environmental official for Rockwell International in Washington, ran unsuccessfully for Congress as an independent in 1972. Elected to the Virginia legislature in 1973, Harris led balloting in last year's GOP legislative primary in the 19th Legislative District.

Thoburn, a self-proclaimed millionaire, made his first bid for the Republican congressional nomination two year ago and lost by less than 200 votes.

Each of the three candidates was asked by The Post complete a questionaire discussing various issues. The candidates' responses on the next page.