Fairchild Industries Inc. yesterday reported a 17 percent decline in its third-quarter profits, blaming it on lower sales volume because of the end of its A10 Thunderbolt II aircraft program.

Fairchild said its net income in the third quarter declined to $7.5 million (40 cents a share) from $9 million (48 cents) in 1983, and revenue slumped to $201.8 million from $211.4 million.

The Germantown, Md., aircraft manufacturer delivered eight Thunderbolts to the Air Force in the third quarter of 1983, and delivered the last of the planes before this year's third quarter.

For the first nine months, Fairchild's earnings were up 37 percent to $23.4 million ($1.24 a share) from $17 million (91 cents) in the 1983 period. Sales increased to $630.8 million from $636.4 million a year ago.

Fairchild said its commercial aircraft sales are lower this year, despite a boost from sales of the new Saab-Fairchild 340. The company's electronics and telecommunications operations continued to grow, it said.

Sovran Financial Corp. reported a 24 percent increase in third-quarter profits, to $21.1 million ($1.21 a share) from $17 million ($1.02) in the same quarter last year.

For the first nine months of 1984, profits rose 14 percent to $59 million ($3.45) from $52 million ($3.11) the year before.

The Norfolk bank company attributed the earnings growth to increases in net interest income and other operating income, as well as a cost control program.

Total assets on Sept. 30 were $7.5 billion, 6 percent higher than a year ago.

Consolidated Rail Corp., or Conrail, the government-owned freight railroad, reported a 64 percent increase in third-quarter profits, to $140.8 million ($5.21 a share) from $85.6 million ($3.21) in the same quarter last year.

Operating revenue rose to $843.7 million from $760.3 million the year before.

In the first nine months of this year, earnings rose 110 percent, to $410.6 million ($15.20 a share) from $195.5 million ($7.31) last year. Revenue increased to $2.6 billion from $2.3 billion a year ago.

The U.S. Department of Transportation wants to sell Conrail and has narrowed its list of potential buyers to three -- Norfolk Southern Corp., and Allegany Corp., both railroad holding companies, and an investor group led by J. W. Marriott Jr., president of Marriott Corp., the Bethesda hotel and food service company.

Suburban Bancorp reported that its profits rose 7 percent in the third quarter, to $6.92 million ($1.38 a share) from $6.47 million ($1.35) for the comparable period of 1983.

For the first nine months, profits at the Bethesda-based bank holding company were up 17 percent, to $19.17 million ($3.94) from $16.37 million ($3.42). As of Sept. 30, total assets had climbed to $2.69 billion, a 14.4 percent increase for the year. The company cited two major factors in the strong showing: an increase in both interest and non-interest income, and success in containing costs.