Bethlehem Steel Corp., citing record import levels and reduced shipments, yesterday reported a third-quarter net loss of $17.4 million (50 cents per share), on sales of $1.4 billion.

Also yesterday, Greyhound Corp. reported a 16 percent increase in net income for the third quarter.

Bethlehem, the nation's No. 3 steel maker reported modest operating income for the quarter as well as reduced operating losses in its basic steel business.

In the third quarter of 1983, Bethlehem posted a net loss of $42.9 million ($1.03), on sales of $1.2 billion.

For the first nine months of 1984, Bethlehem incurred a net loss of $48 million ($1.40), compared with a net loss of $136.1 million ($3.23) in the 1983 period.

"Record levels of steel imports combined with customers' concern about a strike by automobile workers resulted in a sharp reduction in production and shipments during the third quarter," company Chairman Donald Trautlein said in a statement.

Looking ahead, Trautlein said a "surge" in steel imports to more than 30 percent of the domestic market during the quarter "has caused us to lower our estimate of 1984 domestic industry shipments from 77 million to about 74 million tons. We now expect that Bethlehem will incur a loss for the fourth quarter and for the year 1984."

Operating income for the third quarter was $2.7 million, compared with an operating loss of $54.2 million for the third quarter of 1983. For the first nine months of 1984, Bethlehem had operating income of $8.9 million, compared with an operating loss of $286.8 million in the year-earlier period.

The company's basic steel segment had operating losses of $13.4 million and $28.3 million for the third quarter and first nine months of 1984, respectively, compared with operating losses of $40.6 million and $220.4 million for the same periods a year ago. The reduced losses resulted from increased profitability in several product lines, Bethlehem said.

Bethlehem's directors declared dividends of 15 cents per share of common stock, $1.25 per share of the $5 cumulative convertible preferred stock, and 62 1/2 cents per share of the $2.50 cumulative convertible preferred. Each are payable Dec. 10 to holders of record Nov. 9.

Phoenix-based Greyhound said net income for the quarter ending Sept. 30, 1984 was $42.2 million (87 cents per share), compared with $36.3 million (76 cents) for the same quarter last year.

For the first nine months of the year, net income rose to $88.5 million ($1.81), compared with $81.3 million ($1.73) for the same period in 1983.

Revenue for the nine months was $1.6 billion, compared with $1.7 billion for the same period a year ago.

John W. Teets, chairman of the board and chief executive officer, said he was pleased with the third-quarter earnings, which "indicate that the management actions taken over the last year to become leaner and more profitable are beginning to take hold."

All sections of the company showed improvement except for the transportation group and the services and food service section.

Teets said competition from low-cost airlines undercut Greyhound Lines' pricing structures for intercity operations.