United Airlines appears to have earned more money in the third quarter than any airline has ever earned in a full year.

Things are going so well, and United officials are so optimistic that the trend will continue, that they announced yesterday that they have acquired options on 69 more Boeing jetliners that could cost the company $2.2 billion.

Citing record-breaking passenger traffic - in large part stimulated by discount fares - United reported third-quarter earnings yesterday of $166.1 million, up from a restated $82.1 million in the same quarter a year ago.

Delta Air Lines is believed to hold the profit record for a year: $131.1 million in fiscal 1978.

UAL Inc., United Airline's parent company, reported earnings of $170.6 million ($6.74 a share) for the third quarter, up 102 percent from a restated $84.6 million ($3.39) in the 1977 period. UAL, whose subsidiaries also include Western International Hotels and GAB Business Services, earned $135.1 million last year.

UAL's quarterly consolidated operating revenues climbed 23 percent in the third quarter to $1.1 billion from $911.6 million in the year-earlier period.

United officials said that low fares accounted for more than half its airline business in both July and August, but also cited traffic gains attributable to a Northwest Airlines strike and operating efficiencies achieved by changes in aircraft, flight and crew schedules put in place in June for the third quarter earnings gain.

Airline operating revenues were up 23 percent to $1 billion in the third quarter from last year's $818.9 million.

For the nine months, United earnings reached a record $271.5 million on revenues of $2.6 billion against restated earnings of $75.6 million on revenues of $2.2 billion in the year-earlier period.

UAL's consolidated net earnings rose to a record $285.7 million ($11.39 a share) on revenues of $2.9 billion during the first nine months of this year from a restated $90.9 million ($3.64) on revenues of $2.4 billion a year ago.

The options announced yeaterday cover 37 next-generation Boeing B-767 widebodies, at a cost of $1.7 billion for the aircraft and related spare parts and equipment, and 32 more Boeing B-727-200 series jets, at a total cost of $500 million. The 767 options are on planes for delivery between 1982 and 1986, while the 727 options are for delivery in 1981 and 1982.

Continental Airlines reported sharply higher third-quarter profits while Northwest Airlines reported a drop in earnings caused by a pilots strike that shut down the airline for half the quarter.

Continental, which is seeking Civil Aeronautics Board approval of a merger with Western Airlines, said earnings rose to $16.9 million ($1.12 a share) from $12.6 million (87 cents) in the same quarter last year.

Northwest, whose 109-day pilots strike ended Aug. 15, reported earnings of $11.5 million (53 cents a share), down from $29.4 million ($1.36) in the same period of 1977.

Without a "Star Wars" to market this year, 20th Century-Fox Film Corp.'s latest three-month net profit was $18.4 million, down 42 percent from a year earlier.

Fox's earnings in the quarter anding Sept. 30 were $2.26 a share compared with $4.02 last year. Revenues in the last three month period were $161 million compared with $171.5 million in the same 1977 quarter.