Ransome Airlines said it will begin three daily, round-trip flights between Dulles International Airport and New York's LaGuardia Airport next month.

Since Republic Airlines ended its three-times daily service from Dulles to LaGuardia two years ago, promoters of Dulles have been seeking an airline to re-start the route. Those promoters say LaGuardia is the top-requested destination in surveys of potential Dulles users.

In contrast, nearly three dozen nonstop flights are scheduled daily between National and LaGuardia or Newark airports.

Ransome, the nation's largest commuter airline, will use 50-seat de Havilland Dash 7s--quiet, four-engine Canadian airplanes that can take off and land on short runways. The airline will charge $69 each way, or $96 round trip for tickets purchased seven days in advance. The service will begin Aug. 15, with another flight added on Sept. 1.

"If the response is strong enough, we'll add more service at Dulles," Ransome President J. Dawson Ransome promised yesterday. "We frankly view National Airport as a dead-end street for us," he said, citing restrictions on operations and ground congestion there.

A former Allegheny commuter, Ransome will need to fill between 45 and 50 percent of its seats to break even on the Dulles-New York route, he said, adding that he is willing to give it time. "You have to give a market a little time to mature," he said.