MOST PASSENGERS arriving for flights at Washington's three airports are either dropped off by relatives or friends, or they drive their own car and park it at the terminal, a recent survey shows.

But sometimes it is hard to find a friend who has a weekday morning free to deposit even a best buddy at a check-in counter. Family members may have other obligations more important than chauffeuring. And in a one-car family, it could be highly inconvenient to the stay-at-homes if that car is left sitting in the airport lot for several days.

Whatever the reasons for needing them, a variety of alternatives--taxi, bus or van, private limo, rail--is available to get to and from National, Dulles and Baltimore/Washington airports.

Costs of the services vary, depending on the distance and the number of people in the party. For groups of three or less, the airport bus may be the cheapest way to go. But larger groups may find taxis more economical. Private limousines can be expensive, but in this case the passenger is paying for the additional convenience and comfort.

Sample interstate taxi fares (for non-metered cabs), set by the Washington Metropolitan Area Transit Commission, are $5.70 to National from the Capital Hilton, 16th & K streets NW, $28.70 to Dulles. This is based on a rate of $1.70 for the first mile and 50 cents for each additional half mile. The charge is 75 cents for each additional passenger. The fare to Baltimore/Washington from the District is not similarly regulated, but the current rate is about $35 to $38. (Establish the cost before entering the cab, advises the commission.)

A large family, or two families traveling together, might consider hiring a private limousine service to pick them up at their residences and deliver them hassle-free at the airport. For a six-passenger limo, one company, Executive Limousine Service, charges $41.60 to go to National from most points in Washington, $67.60 to Dulles and $72.80 to Baltimore/Washington.

Metrorail's Blue line, servicing National from points in Prince George's County, the District of Columbia and Arlington, is an inexpensive option. From the Farragut West station, the fare is 90 cents during rush hours and 65 cents at other times for the 25-minute ride. Several Amtrak trains (484-7540) departing Union Station stop at the Baltimore/Washington terminal daily, where shuttle service is provided to the airport. The fare is $6.90; reduced rates are available for family groups.

A surprisingly broad network of scheduled bus and van service to the three terminals also is available. The friend or family member who is unable to drive to the airport might be able to deliver the traveler to the nearest bus pickup point. Here are the major bus and van services to the three airports, and approximate costs:

Washington National Airport:

* From downtown Washington, Airport Limo, Inc. (532-1000) operates bus service on a route that includes the Sheraton Washington, Shoreham, Washington Hilton, Mayflower and Capital Hilton hotels and on to the airport. Departures every half hour, beginning at 6:10 a.m. at the Capital Hilton through 8:10 p.m. One-way adult fare is $3.25 ($3.75 from Sheraton Washington and Shoreham). Children 2-6, half-fare on Airport Limo routes.

Hourly departures are made from the Hyatt Regency and Quality Inn on Capitol Hill from 6:15 a.m. to 8:15 p.m. Fare, $3.25.

* From the Maryland suburbs, there are two Airport Limo routes. One begins at the Howard Johnson's in Wheaton and includes the Holiday Inn, Sheraton Inn, Quality Inn and Econo Lodge in Silver Spring. The other services the Wisconsin-Avenue corridor, stopping at Linden Hill, Marriott Bethesda, Ramada Inn, Holiday Inn Bethesda, Bethesdan, In Town Chevy Chase and Holiday Inn Chevy Chase. Both run hourly, 6 a.m. to 8:05 p.m., with later departures by reservation. Less-frequent departures on Sundays. One-way fare, $6.25.

Beltway Limousine Service offers a reservations-only service to National from hotels in Gaithersburg, Rockville, College Park, Beltsville and New Carrollton. Vans make the route about every two hours, though passengers must phone 24 hours in advance for pickup (622-0700). Adult fare from Gaithersburg and Rockville is $14; from Prince George's County stops, $10. Half-fare for children 5 to 12; no charge for those under 5.

* From the Virginia suburbs, one Airport Limo route begins at Dulles, stopping en route at the Sheraton in Reston; Westpark Hotel, the Holiday Inn, Marriott and Ramada Inn at Tysons Corner; and the Hilton Inn in Springfield. The route is covered 12 times daily (except Springfield, with only 6) from 6:50 a.m. to 11:45 p.m. The adult fare for the entire route is $7.75; less from intermediate stops.

A second route also begins at Dulles, stopping en route at the Marriott Key Bridge, the Rosslyn Westpark, the Rosslyn Holiday Inn, the Arlington Hyatt, and the Holiday Inn, Marriott and Hyatt in Crystal City. The route is covered 12 times daily, beginning at 7:50 a.m. from the Key Bridge Marriott until 10:25 p.m. Adult fare from the Rosslyn area, $2.25; from Crystal City, $1.75.

Dulles International Airport:

* From downtown Washington, Airport Limo operates a bus from the Capital Hilton on the hour from 6 a.m. to midnight. The bus stops 15 minutes earlier at the Washington Hilton, where it takes on shuttle van passengers from the Sheraton and Shoreham. The adult fare is $7.75 (25 cents extra for Sheraton and Shoreham passengers).

Executive Limousine Service (671-0800) provides van service from the Quality Inn and Hyatt Regency on Capitol Hill and the L'Enfant Plaza Hotel each afternoon at 1:50, 2:50 and 3:50 p.m. Adult fare is $10; children under 12, $5.

* From the Maryland suburbs, Airport Limo makes eight runs daily from Wheaton and Silver Spring between 7 a.m. and 10:15 p.m., stopping at the Sheraton, Holiday Inn and Howard Johnson's hotels.

Eight additional runs are made from Bethesda and Chevy Chase between 8 a.m. and 9:30 p.m., stopping at the Linden Hill, Marriott, Ramada Inn, Bethesda Holiday Inn, Bethesdan, In Town, and Chevy Chase Holiday Inn hotels. Adult fare, $8.75.

* From the Virginia suburbs, Airport Limo reverses its two Dulles to National routes. Twelve departures from National to Dulles from 6:50 a.m. to 11:25 p.m. via Springfield, Tysons Corner and Reston; departures from 7 a.m. to 9:50 p.m. via Crystal City and Rosslyn. Fares as described above.

Baltimore/Washington International Airport:

* From downtown Washington, Airway Limousine Service makes 24 bus runs daily from the Capital Hilton between 6:15 a.m. and 3:20 a.m. Four additional van runs are made daily from the Hyatt Regency on Capitol Hill, at 5:30 a.m., 11 a.m., 3 p.m. and 6:30 p.m. Adult fare is $7.50; children 6-11, $2.

* From Maryland suburbs, Airway Limousine provides pickup service from home, office, hotel or Metro stops in Prince George's and Montgomery counties. At least three to four hours advance notice is normally required (441-2345); to guarantee pickup, reserve a day in advance. The fare is $11 to $13, depending on distance. For groups of two to five the charge is $16.50 and $20.

Reservations can also be made for a pickup at the "kiss and ride" Beltway Limo Terminal at the National Guard Armory in Greenbelt--$5.25.

Travelers who find parking their cars at the airport the handiest way to catch a plane might consider the costs at each terminal when booking their next flight, since long-term parking fees differ.

At National, the minimum charge is $5 for a 24-hour day. At Dulles, the rate is $2 a day. At Baltimore/Washington, the "daily" rate is $4.50 after the first day. For five days or more, BWI's "valet" parking is more economical: $21 for five days, $2 more for each additional day.