Norfolk Southern yesterday announced a 5 percent increase in third-quarter profits and restated a threat to withdraw its bid to buy Conrail because of congressional inaction on the firm's proposal for the government-owned freight railroad.

"Time for talking about Conrail is over," said Robert B. Claytor, Norfolk Southern's chairman and chief executive, in a meeting where he released the company's quarterly profit statement.

Claytor said his company would abandon its $1.2 billion contract with the Department of Transportation unless Congress gives a clear indication that it is moving to approve the sale. The offer expires at the end of the year.

The company reported that profits rose 5 percent in the third quarter on the strength of the sale of shares in Santa Fe Southern Pacific Corp.

Net income for the third quarter totaled $141.6 million ($2.25 a share) compared with $134.7 million ($2.14) for the third quarter in the previous year. Third-quarter results included a gain of $19.7 million (31 cents) from the sale of 1.7 million shares in Santa Fe Southern Pacific.

In the year to date, the company said net income was $380.1 million ($6.04), up 1.6 percent from the first nine months of 1984, when the company earned $374 million ($5.94).

*Media General Inc., the Richmond-based communications company, yesterday reported a lower third-quarter profit of $6.8 million (95 cents a share) on revenue of $139 million, versus a profit of $8.9 million ($1.25) on revenue of $132.7 million in the same period last year.

"Two of our business segments continue to depress overall earnings," said Media General President James S. Evans. "Media General Broadcast Services is still operating at a loss while our significant restructuring and rebuilding efforts continue.

"And our cable television system in Fairfax County, Va., is in the third year of its four-year construction and start-up phase," Evans said. "Progress toward completing the system is encouraging, with 76,000 subscribers being served and cable now passing 159,000 of the 240,000 homes available."

For the first nine months, Media General reported lower net income of $24.1 million ($3.39) on revenue of $426.3 million versus net income of $26.6 million ($3.74) on revenue of $404.3 million in the same period last year. Those numbers exclude $546,000 of nonrecurring charges in 1985 and a nonrecurring gain of $2.7 million in 1984. Including these items makes net income for the nine months this year $23.6 million ($3.31) versus $29.3 million ($4.13) in the comparable period last year.

*Computer Entry Systems Corp. of Silver Spring reported third-quarter net income of $506,000 (12 cents a share), up 15 percent from earnings of $439,000 (11 cents) in the same period of the prior year.

Revenue increased to $9.9 million from $7 million for the company, which manufactures cash-management and payment-processing systems for banks.

For the first nine months, profits rose 36 percent to $1.2 million (30 cents) from $913,000 (23 cents) the year before. Revenue increased to $27.7 million from $18.9 million last year.

ERC International of Vienna reported third-quarter profits of $725,851 (19 cents a share), up 103 percent from $358,433 (10 cents) the year before. Revenue rose to $19.2 million from $9.7 million.

For the first nine months, net income rose 23 percent to $1.6 million (41 cents) this year from $1.3 million (33 cents) last year. Revenue rose to $44.2 million from $28.7 million.