Bethelehem Steel Corp. said yesterday that it earned $76.3 million ($1.75 a share) in the third quarter in contrast with a loss of $32.3 million a year earlier.

Sales spurted to $1.654 billion from $1.479 billion, raw steel output jumped 50 percent and production capacity utilization climbed to 77 percent from 54 percent a year ago.

Chairman Donald H. Trautlein, however, said the higher demand that produced turnarounds for the third quarter and first nine months was followed by a slowdown in the order rate, which augurs for much less satisfactory gains in the final quarter.

Nine-month net income was $179.8 million ($4.12) on sales of $5.659 billion compared with $65.2 million ($1.49) a year earlier on sales of $5.051 billion.

Trautlein observed that imported steel products captured 19 percent of apparent domestic consumption in July and 25 percent of consumption in August.

He said Bethlehem's shipbuilding and repair operations had better earnings in the third quarter than last year due to demand for offshore drilling rigs and U.S. Navy ship repair contracts. Kusan Inc., the plastics subsidiary, also made a better showing.

Bethlehem's directors declared the regular quarterly dividend of 40 cents a share payable Dec. 10 to holders of record Nov. 10.

Citing reduced deliveries of some aircraft, Boeing Co. reported yesterday that its earnings dropped from 1980 levels in the third quarter and first nine months.

Boeing said it earned $92.5 million (96 cents a share) in the quarter compared with $139.8 million ($1.45) a year earlier. Sales were unchanged at $2.22 billion.

In the first nine months, Boeing earned $377 million ($3.91), against $434.8 million ($4.51) in the comparable 1980 period. Sales rose to $7.41 billion from $6.77 billion.

Consolidated Rail Corp.yesterday reported a third-quarter profit of $64.9 million (42 cents a share), only the third quarterly profit in its five-year history.

The East Coast railroad, created out of six bankrupt railroads by Congress as a for-profit, private corporation, had posted a loss of $88.1 million in the third quarter of 1980.

In the 1981 second quarter, Conrail earned $13.8 million. Thus Conrail reported its first back-to-back quarterly profit since it began operating April 1, 1976. Its only other profitable quarter was in the second period of 1979 when it reported income of $23.3 million.

In the first nine months, Conrail reported net income of $12.8 million ($2.56) on consolidated revenue of $3.2 billion compared with a 1980 nine-month net loss of $231.8 million on revenue of $2.9 billion.

L. Stanley Crane, chairman and chief executive, attributed the turnaround to cost-cutting, including reductions in the size of the Conrail work force and physical plant, and wage sacrifices agreed to by some union and nonunion employes.