The battle this fall for the three delegate seats in Virginia's 20th District - Prince William and Loudoun counties - should provide a classic measurement of the values of party unity and incumbency.

Three Democrats - Floyd C. Bagley, Earl E. Bell. and David G. Brickley - who are completing their first terms in the Virginia House have merged their records of constituent service and experience.

Three Republican challengers - Paula Faraday, Robert Statz, and Eileen Stout - are attacking that unity as "machine politics," and Faraday and Stout are swinging away so freely that the race could become bitterly personal before the Nov. 8 election.

A seventh candidate, C. Mason Gardner of the American Party, is sticking to issues and is described by his opponents as a "gentleman" as he attempts to prove that a third party candidate can win in Virginia.

Recently at one of more than a dozen joint appearances of all three parties across the two-county district, Bell said. "Mr.Statz is a gentlyman and Mr. Gardner is a gentlyman, but I'm a little disappointed with the sort of campaign the ladies are trying to run."

Bell, a Leesburg car dealer, was upset at being called "a used car salesman from Loudoun County" by Stout. He said in an interview that he had spoken earlier to Stout and Faraday "on a personal basis and told both that we have always campaigned on a high level and don't want to get into name calling."

But at the candidates night with the Dale City Civic Association. Farday fired back in a prepared speech: "Mr. Bell, last week you told me to back off.' Well, Mr. Bell, I have no intention of backing off.'

"The one for all and all for one approach of the incumbents is an approach that cheats the electorate," Farraday said.

During the question and answer session, a number of pointed questions for the Democrats came from GOP campaign workers. Brickley, a past president of the Dale City Civic Association, later said. "I know everybody from Dale City and we had some ringers there."

"I hope the other appearances get no meaner," he said. "The two ladies are good at calling names, but we will not retaliate in kind. I can assure you of that."

Stout, who has called Democrats "the three stooges," described the incumbents as "sensitive. They're not used to being effectively opposed. I don't mean to attack a person, but there are convictions involved."

While Gardner is concentrating on attacking the constitutionality of the state bond issue and hoping to get enough votes to squeeze into third place, the major "issue" in the campaign has become "the three B's," as the Democrats call themselves.

They wear identical ties dotted with little maps of Virginia, plan to use joint radio spots in the last stages of the race, and when asked questions usually reply, "We..."

They say they have been successful in solving the problems of hundreds of constituents and in presenting a unififed face in Richmond and say that they have experience that their Republican foes lack.

They describe themselves as moderates, back the party's state ticket, and frequently mention the name of Sen. Harry F. Byrd Jr.

Bagley, Bell, and Brickley say that their joint approach is working well, that haviang the only Loudoun County candidate - Bell - on their ticket is a big advantage and that they expect to get 60 per cent of the vote.

The Republicans say that the unified approach was effective at first but that they have countered it by hammering away at the need for three distinct voices in Richmond.

They attack the Democrats as spenders who voted themselves a big pay increase. (The Virginia assembly raised pay from $5,400 to $8,000 effective in 1980.)

The GOP candidates say they are so diverse that everyone will be able to find someone to represent him or her self. They range from the conservative Faraday, who is active in the fight against the Equal Rights Amendment, to the moderate Statz, who opposes higher taxes (as do all the candidates), to Stout, who says that public employees should have the right to meet and confer with employers in contract talks.

Faraday, Statz, and Stout say their individual campaigns are going well and that they can expect to knock off the Democrats.

Gardner, who campaigns with frequent humor and says his secondary goal is to get his party's platform before the public, says he believes he can top the 6.542 votes he got two years ago and win a seat for the American Party.

The candidates: American Party

C. Mason Gardner, 43, of Gainesville, operates a mobile home and camper sales and camp business. A resident of Prince William County for 31 years, he was born in Washington and attended the University of Maryland. He is a member of the John Birch Society and the National Rifle Association. Democratic Party

Floyd C. Bagley, 55, of Dumphries, was born in Maine and came to Virginia at 17 to join the Marines. He won a battlefield commission in 1944, rewon his commission in the Korean War, graduated from law school while in the Marines, and retired as a captain in 1959. He served as Marine Corps military judge in 1970-71 and as Prince William County attorney from 1972 until his election in 1975.

Earl E. Bell, 57, owns the Chrysler-Plymouth dealership in Leesburg and lives near Hamilton. A native Virginian, he had been active in a wide range of civic organizations before making his first, and succesful, run for political office in 1975.

David G. Brickley, 33, of Woodbridge, came to Virginia in 1970 as an Air Force intelligence officer and was assistant county executive in Prince William County in 1974-75. He is vice president of an investment company. Republican Party

Paula C. Faraday, 47, of Catharpin, is a native of Maine who came to Washington 14 years ago and was director of student acctounts at Georgetown University for five years. She has lived in the rural area west of Manassas for the last five years and has been active in public speaking against the ERA and has worked on GOP campaigns.

Robert Statz, 30, was born in Cleveland and is a graduate of Ohio State University. He is a captain in the Army Reserves and works as an independent insurance agent, real estate agent, and tax consultant in Manassas.

Eileen Stout, 32, of Dumfries, is a member of the town council there and is a teacher in the Fairfax school system. She was born in North Carolina and raised on Long Island. She is a teacher of French and Spanish and has a masters degree in secondary school administration.