About three-quarters of the nation's scheduled airline flights got off the ground yesterday in the third day of the air traffic controllers' strike, the Federal Aviation Administration said. Airlines reported that slightly more seats were occupied, which they attributed to rising public confidence.

Passengers using bottleneck airports continued to suffer serious delays yesterday. Eastern Airlines' New York shuttles were as much as two hours late taking off from LaGuardia Airport.

According to Transportation Secretary Drew Lewis, by later afternoon the average departure delay at airports in the East was 23 minutes, and in the West 17 minutes.

Airline spokesmen continued to say planes could accommodate almost everyone who wanted to fly.

Major carriers reported their planes were taking off between 55 and 70 percent full, on the average. "If you want to travel, there's space," said Tom Germuska, spokesman for United Airlines, which flew about 760 of its normal 1,170 daily departures. Air freight companies also said that packages were moving despite the strike.

Yesterday the FAA's emergency "flow control" measures smoothed out sufficiently to allow restoration of some services suspended when the strike began Monday morning. Baggage again was being transferred between airlines, for instance, and some carriers were accepting unaccompanied children and perishable freight goods.

National Airport, the busiest of the Washington area's three facilities, handled about 70 percent of its normal 1,200 daily takeoffs and landings by commercial planes and private aircraft, according to a tower official.

Airline officials there said that Boy Scouts returning home from the national jamboree at Fort A.P. Hill, Va., near Fredericksburg, were helping to fill to capacity many flights that were half empty on Tuesday. About 6,500 scouts departed yesterday from the three area airports.

Through the day, chartered buses arrived at National, disgorging uniformed boys and mountains of packed-up outdoor gear -- tents, sleeping bags, cooking utensils. A few scouts, however, took the precaution of keeping sleeping bags close by in case they had to pass a night at the airport.

But yesterday, most scouts left on time, with help from airline schedulers. Eastern restored the canceled Flight 663 from Washington to Atlanta after it was learned that more than 60 scouts headed for Las Vegas would miss their connections without it.

Other passengers found that travel was possible, but only with some juggling. Ken Bedore, a Navy flight engineer based at Patuxent River, Md., learned yesterday morning that one leg of his Washington-Chicago-Wichita itinerary on United had been canceled, but with phone calls located a seat on TWA.

For most travelers the controllers' strike has meant inconvenience at worst. But for many people employed at National and other airports, the reduction in passengers has drastically lowered their earnings. "Everybody's anxious for them to come back to work," said taxi dispatcher Aubrey Epps, watching a long line of idle cabs. "It's taking money out of everybody's pocket."

Cab Driver Joseph Turary said that on a normal day he ran four or five trips into town by 1 p.m. But yesterday, at 2:20 p.m., he had only two fares and still was waiting. Skycap supervisor Brian Grenadier said he typically took in $60 to $80 a day in tips. "Yesterday I made $23," he said.