Washington area gasoline prices have plummeted an average of 6.2 cents a gallon since December and now are lower than they have been in nearly three years, according to a survey conducted yesterday by the American Automobile Association.

Gasoline has broken the dollar-a-gallon barrier and is selling for 99.9 cents a gallon for self-service regular at five stations in the survey. One of those stations is located in the District of Columbia, and the other four are in Maryland. None of the Virginia stations sampled had gasoline for less than $1 a gallon.

The average for all grades of gasoline at 101 self- and full-service stations sampled is $1.273 a gallon compared with $1.335 on Dec. 14, the survey said. The last time prices were this low was in April 1980 when the AAA survey average was $1.28 a gallon for all grades.

In this latest check, AAA found prices were lowest in Maryland, where the average was $1.252 a gallon, and highest in the District of Columbia, where the average was $1.298. Virginia ranked between those two with an average of $1.277.

"These lower prices for gasoline and diesel reflect the soft oil prices internationally and the price wars that are going on locally," said Tom Crosby, director of public affairs for the Potomac division of the auto club, which conducted the survey.

The AAA survey also uncovered these gasoline price trends:

* Diesel prices also have decreased, but not as much as gasoline. The average price for diesel now is $1.31 a gallon, down only 4.7 cents from December, while gasoline was down 6.2 cents a gallon.

* Motorists can save as much as 60 cents a gallon by checking prices and pumping their own fuel. One station in the survey, for example, was selling full-service regular gasoline for $1.599, compared with 99.9 cents a gallon for self-service regular at five other stations.

* Over the past two years, the prices of gasoline in the metropolitan area have shown dramatic swings, rising from an average of $1.28 a gallon in April 1980 to a high of $1.454 a gallon in April 1981 and then gradually sliding down. Last May, prices hit $1.299 a gallon and then began climbing again, rising to $1.38 last August. Then they started dropping again. In the last six months area gasoline prices have decreased 10.7 cents a gallon.

Actual prices of gasoline vary widely, depending on the brand, the grade of fuel, whether it is pumped by the motorist or the station attendant, and what the local taxes are in the jurisdiction in which the station is located. But AAA officials believe their survey, based on a random sampling of stations, represents an accurate measure of the average price of gasoline and of the trend of gasoline prices.

Crosby said that price wars have broken out in pockets where there are clusters of highly competitive station operators. Some of the price-war pockets include the District Heights area of Maryland, the strip of Rte. 50 between Arlington and the Beltway in Virginia, and the cluster of stations near Pennsylvania and 12th Street SE in the District of Columbia.

"People are more cost-conscious today, and when a dealer sees a stream of cars pulling into a nearby station where prices have been lowered, then he begins to lower his prices," Crosby said.

He said that today's reduced prices are "great for motorists," but cautioned that the long-range result could create new problems.

"The lower gas price puts pressure on the full-service station--the guy whose margin is shrinking and who may be driven out of business if this keeps up for a long time," Crosby said.

"And that leaves the motorist with fewer full-service stations where he can go to have the battery checked, the oil changed and minor and major repairs made."