USAir Group Inc. yesterday reported that its third-quarter earnings dropped by nearly $10 million over the same period a year ago, partially because of the airline industry's price wars.

The decline was reported despite a jump in revenue of $39 million over 1984 quarterly figures.

The holding company for USAir Inc. airline said its earnings fell about 26 percent to $25.9 million (89 cents a share) on revenue of $452.1 million, compared with $35.1 million ($1.30) on revenue of $413.4 million during the same period a year ago.

"Results for the quarter reflect the decline in the rate of traffic growth following the Labor Day holiday, combined with continued heavy discounting prevalent throughout the industry," said Edwin I. Colodny, USAir chairman and president.

For the first nine months, USAir said its earnings dropped more than 7 percent to $85.4 million ($2.92) on revenue of $1.3 billion, compared with $92.6 million ($3.46) during the same period in 1984.

* Atlantic Research Corp., the nation's biggest maker of small rockets, reported that its net income for the third quarter increased 21 percent to $3.77 million (51 cents a share) from $3.12 million (42 cents) in the same period a year ago.

The Alexandria-based company said its revenue jumped to $59.5 million, compared with $55.8 million during the same period in 1984.

For the first nine months, the company's net income climbed to $11.48 million ($1.55) on revenue of $167.48 million, compared with $8.55 million ($1.16) on revenue of $148.9 million during the first nine months of 1984.

A net after-tax gain of $1.1 million (15 cents) from the sale of Atlantic Research's former subsidiary, Tri-Seal International, is included in the nine-month earnings.

* NS&T Bankshares Inc., parent of NS&T Bank, reported a 27 percent increase in net income during the third quarter of 1985 compared with the same period last year.

Net income was $2.1 million ($1.54 a share)) for the latest quarter, compared with $1.7 million ($1.21) last year.

For the first nine months of 1985, net income totaled $6.3 million ($4.61), a 35 percent increase over $4.7 million ($3.42) earned for the same period a year ago.

The increased earnings were attributed to a 12 percent increase in interest-earning assets and an improvement in their interest margin.