Fairchild Industries Inc. reported yesterday that third-quarter earnings rose to $12.5 million (98 cents a share) compared with earnings of $11.3 million (90 cents) for the same period in 1979.

The aerospace and communications company said that earnings for the first nine months of the fiscal year were $37 million ($2.90) on sales of $621 million. During the first nine months of 1979 earnings were $28.1 million ($2.27) on sales of $510 million.

Fairchild said that sales for previous periods have been restated to reflect the partnership, finally agreed to in the third quarter, of Fairchild and Continental Telephone Corp. The two companies share equally in American Satellite Corporation.

Another major development in the third quarter was an agreement reached in September to acquire VSI Corp., a $400 million producer of aerospace fasteners, tooling for the plastic industry and other products and delivery of 22 Merlin and Metro aircraft by Fairchild's Swearingen Aviation Corp.

Martin Marietta Corp. reported third-quarter net earnings of $51.3 million ($2.06 a share) on sales of $697.4 million compared with earnings in the third quarter of fiscal 1979 of $54.1 million ($2.16) on sales of $530.4 million.

Earnings for the first nine months of 1980 were $144.6 million ($5.81) on sales of $1.908 billion compared with $140.7 million ($5.60) on sales of $1.491 billion in 1979.

Computer Data Systems Inc. announced net income for the three months ended Sept. 30 of $249,000 (44 cents a share) on revenues of $4.1 million compared with $152,800 (28 cents) on revenues of $3.1 million in the comparable period last year. Chairman Clifford M. Kendall said that growth in programing and processing services offered by the company contributed to the increase in revenues.

Income from continuing operations was $239,200, which exceeded last year's earnings of $143,000.

Ethyl Corporation of Richmond said that net income declined in the third quarter of 1980 to $19.2 million (94 cents a share) on sales of $430 million compared with income of $28 million ($1.38) on sales of $442 million in the third quarter of 1979.

Chairman F.D. Gottwald Jr. said he was disappointed with the results but expects to see some improvement in the forth quarter. Even so, he said he expects earnings for 1980 to fall below record 1979 results.

Net income for the first nine months of 1980 was $65 million ($3.18) compared with $75.51 million ($3.70) in 1979. Results included a first-quarter after-tax capital gain of approximately $7.3 million (37 cents on a fully diluted basis) from the sale of a tract of land in Louisiana and a second-quarter after-tax charge of about $6.8 million (35 cents) for the shutdown of an antiknock facility at Houston.