Continuing on a profitable path that many other airlines would envy, USAir yesterday reported record earnings for the third quarter and the first nine months of the year.

The Washington-based airline reported net earnings of $15.4 million ($1.19 a share) in the third quarter, a 22 percent increase over the previous record earnings of $12.7 million (98 cents) in the third quarter of 1979, a period that included a net gain of $2.3 million on disposition of flight equipment. Total revenues for the third quarter were $249.5 million compared, with $195.2 million in the same quarter of 1979.

USAir also reported record nine-month earnings of $42.1 million ($3.28) on revenues of $704.5 million compared with earnings of $24.3 million ($1.94) on revenues of $522.9 million in the first nine months of last year.

September figures were also up: Net earnings were $3.1 million on revenues of $78.9 million compared with earnings of $2.2 million on revenues of $61.4 million in September 1979.

Hinting that the airline's impressive performance so far this year would continue, Edwin I. Colodny, USAir's chairman and president, said yesterday that October traffic has been strong and that early bookings are heavy for the Thanksgiving and Christmas holiday periods.

Wire services also reported the following transportation company earnings:

Union Pacific Corp., the nation's third-largest transportation company, said its net income rose 38 percent to $118.1 million ($1.24 a share) from $85.5 million (89 cents) in the same period last year.

Union Pacific Corp. Chairman James H. Evans said increased coal traffic was a major reason for his company's higher earnings. He said revenues jumped 21 percent to $1.19 billion from $976.9 million in the third quarter last year.

Nine-month earnings rose 6 percent to $299.4 million ($3.13) from $281.7 million ($2.95). Revenues were up 24 percent to $3.54 billion from $2.86 billion.

Southern Pacific Co., a diversified company whose holdings include transportation, land, title insurance and communications operations, reported a 74 percent jump in profits for the third quarter to $53.5 million ($1.99 a share) from $30.8 million ($1.15) a year ago. Revenues climbed to $727.4 million from $672.9 million.

Nine-month net income was $91.57 million ($3.40), 26 percent below 1979 nine-month net income of $123.08 ($4.57). Revenues rose 9 percent to $2.106 billion from $1.936 billion.

Southern Pacific said railroad operations recorded operating income of $32.58 million in the quarter compared with a $4.79 million loss a year earlier.

Seaboard Coast Line Industries Inc., which is expected to merge with Chessie System Inc. to form a new transportation company, said its profits rose 51 percent to a record $46.6 million ($3.10 a share) from $30.8 million ($2.06) last year and revenues jumped to $610.8 million from $536.1 million.

Nine-month net income also was a record $125.4 million ($8.37) compared with $87.3 million ($5.85) a year earlier. Revenues rose 16 percent to $1.84 billion from $1.59 billion.

Seaboard Chairman Prime F. Osborn said shipments of coal during the third quarter were "very strong." Those gains more than offset declines in shipment of items such as automobiles, construction materials, phosphate chemicals and steel.