* USX Corp. reversed last year’s third-quarter...

* USX Corp. reversed last year’s third-quarter loss, mirroring a turnaround in its steel business that the company attributed to higher prices and more shipments.

USX reported a $66-million profit, against a $4-million loss a year ago.

Its U.S. Steel business had a profit of $36 million, or 44 cents a share, on revenue of $1.43 billion. That compared to a loss of $28 million, or 48 cents a share, on sales of $1.27 billion in the comparable period of 1992.

Meanwhile, USX’s Marathon Oil group saw a 25% profit increase to $30 million, or 10 cents a share, on sales of $2.98 billion. In the 1992 third quarter, Marathon reported profits of $24 million, or 8 cents a share, on sales of $3.38 billion.

* Computer maker AST Research Inc. said it earned $8.2 million in its first fiscal quarter--a slight gain despite much stronger sales.

The Irvine-based firm said earnings for the quarter ended Oct. 2 were 26 cents a share on sales of $514.4 million. AST earned $7.6 million, 24 cents a share, on sales of $286.4 million a year ago.