Tumbling coal and auto sales derailed Norfolk Southern Corp.'s profits for the third quarter this year, knocking the company's net earnings 56.3 percent below profits for the same period in 1981.

Roanoke-based Norfolk Southern is the parent company of the Norfolk & Western and Southern railways, which merged June 1. The third-quarter earnings report reflects the combined profits of N&W and Southern in the year-ago period, and compares those with the earnings of the new company today.

The new company pulled in profits of $61.3 million (97 cents per share) for the quarter, compared with $140.3 million ($2.26) last year. Total third-quarter revenues for the corporation were $770.9 million, a 20.6 percent drop from $970.5 million a year earlier.

Year-to-date net income was $330.6 million ($5.28), a 7 percent drop from $355.3 million ($5.74) in the same period in 1981. Total revenues for the first nine months of 1982 were $2.59 billion, a 2.2 percent decline from $2.65 billion a year earlier.

Robert C. Claytor, Norfolk Southern chairman and chief executive, said the losses reflect "the impact of the recession on the basic industries our system serves, particularly coal, chemicals, automobiles and building materials." The "worldwide slowdown in coal markets" contributed to the steep decline in coal shipments, Claytor said.

Norfolk Southern yesterday also appointed John L. Jones as administrative executive vice president of the company.

In other earnings reports yesterday:

McLean-based Dynalectron Corp. yesterday announced record net earnings for the 1982 third-quarter and year-to-date periods. For the quarter, net earnings were $4.1 million (48 cents), a 12 percent increase over $3.4 million (43 cents) in the year-ago period. Total revenues for the quarter were $117.9 million, compared with $118.7 last year.

The diversified electrical contracting firm's net earnings for the first nine months of 1982 were $9.1 million ($1.07), compared with $9.0 million ($1.06) last year. Year-to-date total revenues were $336.4 million, down from $349.2 million for the same period in 1981.

Falls Church-based UNC Resources Inc. reported a net loss of $4.1 million (37 cents) for the quarter, compared with a net profit of $481,000 (4 cents) in the year-ago period. Revenues for the quarter also declined, to $60.2 million in 1982 from $66.5 million last year.

UNC, a mining and resource development company, reported a loss of $7 million (63 cents) for the first nine months of 1982, compared with net earnings of $8 million (75 cents) last year. Year-to-date total revenues were $269.8 million, compared with $211.4 million in the previous year.

Keith A. Cunningham, UNC's chairman and chief executive officer, said the losses came about because, "The downturns in the oil service and machine tool industries have been both deeper and more prolonged than we anticipated."

Roanoke-based Dominion Bankshares reported a third-quarter net income of $5.2 million (53 cents), a 3.6 percent drop from $5.4 million (54 cents) in the year-ago quarter. Year-to-date net income was $14.7 million ($1.49), a 12.6 percent drop from $16.8 million ($1.70) in 1981.

Baltimore-based Union Trust Bancorp reported a quarterly net income of $3.6 million ($1.48), a 19 percent gain over $3 million ($1.24) in the year-ago quarter. Year-to-date net income was $9.8 million ($4.02), a 17.3 percent gain over $8.4 million ($3.42) in 1981.

Pargas, based in Waldorf, posted a record nine-month profit of $7.9 million ($1.90), compared with $7.7 million ($1.85) in 1981. For the same period, revenues dropped to $193 million from $201 million.

Third-quarter profits jumped to $2.7 million (66 cents), compared with $1.7 million (42 cents) last year. Revenues for the latest quarter climbed to $60 million from $65.8 million.

Isomet Corp. of Springfield posted third-quarter revenues of $926,000, down from $1.3 million in 1981. Profits fell to $94,000 (6 cents) from $251,000 (16 cents) last year. For nine months, Isomet had earnings of $341,000 (22 cents), compared with $625,000 (41 cents) in 1981. Revenues over three quarters were $2.9 million, compared with $3.2 million.

Media General of Richmond had record revenues and earnings for the third quarter and nine months. The company had nine-month profits of $24 million ($3.40) on revenues of $311.5 million, compared with $22.8 million ($3.18) on revenues of $267.3 million. For the latest quarter, Media General posted profits of $7.8 million ($1.11) on revenues of $115 million, compared with $7.7 million ($1.08) on revenues of $88.3 million.

Richmond-based Old Dominion Real Estate Investment Trust reported a drop in its third-quarter profits and in its year-to-date net income. Earnings for the quarter were $54,529 (6 cents), compared with $353,856 (48 cents) last year. Year-to-date net income was $324,137 (38 cents), compared with $914,941 ($1.26) for the year-ago period.

But Old Dominion's total revenues for the quarter were $1.3 million, a 30 percent jump over $961,557 in pretax earnings last year. Year-to-date total revenues were $3.7 million, 30 percent better than $2.8 million for the same period last year.