Looking a bit like high school seniors on prom night, Virginia Gov. Charles S. Robb and Democratic congressional candidates Richard L. Saslaw and John Flannery suited up and piled into a brown convertible yesterday to do a some last-minute courting of Northern Virginia voters.

The eager trio was clearly the last thing that Harriet Prignano and her husband Richard expected to see when they sat down to shrimp salad on a plain bagel at a restaurant in Old Town Alexandria.

"This is marvelous!" Prignano said, eyes wide as Robb, Saslaw, Flannery and a phalanx of police officers, campaign workers and camera crews streamed through the door and crowded around her table. "We didn't have anything like this on Long Island."

Prignano, a new arrival to the Virginia suburbs, sat patiently as the three peered over her booth, smiled and asked questions. After a few minutes, Robb decided they'd interrupted her lunch long enough and led the crowd out the door. As they exited, the startled expression on Prignano's face relaxed into a smile.

Who will she vote for? "Really, it's hard to say," Prignano said slowly. "I'm a Republican. But I do think these gentlemen will be great congressmen because they really seem to give their all."

Four days before the election, politicians are thick as crows on Virginia's highways and byways. So thick, in fact, that it gets hard to keep them straight. "There's a man named . . . named Hyland," Prignano said ruminatively. "He stands out on Rte. 1, I think . . . but I'm not sure he's in my area . . . ."

By the time Prignano stopped talking about Gerald Hyland, a Democratic candidate for the Fairfax County Board of Supervisors and got back to her bagel, Robb and Co. were already zooming out of Alexandria toward Fairfax City.

For Saslaw and Flannery, the 90 minutes from a commuter terminal at National Airport to Old Town and then to Fairfax was a final hurrah after months of campaigning to defeat two well-financed Republican incumbents.

For Robb, a Northern Virginian, it was a chance to visit old stomping grounds and put his shoulder to the Democratic effort.

"The fact that Lynda and I are moving back to Northern Virginia," Robb said, gives him a great interest in both the Saslaw and Flannery races. "I have absolutely no hesitation in endorsing them."

Of the 8th District's Saslaw, Robb said: "I have known and worked with Dick Saslaw, he's handled a lot of really tough legislation in the state Senate and he's shepherded it through. He's respected by colleagues on both sides of the aisle."

Saslaw is opposing Republican Rep. Stan Parris.

Robb, whose home is in McLean, also said that he cast his own vote by absentee ballot a few days ago, and that he had supported Flannery, who is running against GOP Rep. Frank R. Wolf. Flannery "can count on me," Robb said.

Robb remained coy about his own plans when he leaves office in 1986. Democratic party leaders urged him to run against Republican Sen. John W. Warner this fall, but Robb refused.

Crowds yesterday were sparse but appreciative. At National Airport, the group was joined by Arlington Democrat John Milliken, who is seeking reelection to the county board there. In Alexandria, Mayor Charles E. Beatley and Vice Mayor Patsy Ticer were among the Democratic well-wishers. Passers-by picked up campaign literature and a few called "Good luck!"