Profits of E. I. duPont de Nemours & Co. rose sharply in the traditionally weak third quarter, chairman Irving S. Shapiro reported yesterday. Chemicals and plastic sales were up 15 per cent during a period when such business lines normally are sluggish.

At a special stockholders' meeting in Wilmington, Shapiro also revealed plans for his company to sell Delaware's principal newspapers - the morning News, evening Journal and weekend News Journal.

The newspapers long have been owned by the du Pont family's Christiana Securities Co., a Wilmington holding company. Stockholders of both the du Pont Co. and Christiana yesterday approved a proposed merger of the holding company into the nation's largest chemical firm.

Du Pont's third-quarter earnings totaled $143 million ($2.91 a share) compared with $101 million ($2.03) in the same period last year as sales increased to $2.4 billion from $2.1 billion.

That brought nine-month profits to $426 million ($8.64 a share) on sales of $7.1 billion compared with the 1976 earnings of $366 million ($7.43) and sales of $6.3 billion.

United Technologies Corp., of Hartford, Conn., reported a 24 per cent gain in third-quarter profits to $50.6 million ($1.42 a share) compared with $40.7 million ($1.26) in the 1976 period. Sales rose 6 per cent to $1.34 billion.

Chairman Harry J. Gray said United has continued to enlarge its industrial and commercial business, which accounted for 69 per cent of total sales in the first nine months. Government sales rose by 11 per cent.

For the first nine months, profits totaled $146.4 million ($4.30) vs. $117.9 million ($3.87), a gain of 24 per cent. Sales were up 8 per cent to $4.1 billion.

Philip Morris Inc., the nation's second-largest producer of cigarettes, reported significantly higher third quarter earnings.

The New York-based firm, which accounts for about one-quarter of all U.S. cigarette production, said earnings for the period were $94.14 million ($1.57 a share) compared with $74.54 ($1.25) in the third quarter of last year. Revenues for the period were $1.37 billion, compared with $1.12 billion.

For the first nine months of this year, the firm earned $250.7 million ($4.19) compared with $198.6 million ($3.34). Revenues were up to $3.84 billion from $3.13 billion.

Two large New York bank firms reported modest gains in third-quarter profits. Bankers Trust New York Corp. earned $15 million ($1.25 a share) in the latest quarter compared with $14.3 million ($1.18) a year earlier.

The earnings, before securities transactions, reflected increased income from international business as well as higher domestic commission and other income, an official of Bankers Trust said.

Profits before transactions for the nine months of 1977 totaled $42 million ($3.46), an increase from $41 million ($3.37) during the 1976 period.

Manufacturers Hanover Trust Co., the holding company for the nation's fourth-largest bank, reported third quarter earnings before securities transactions of $39.1 million ($1.27) a share up from $37.5 million ($1.26) a year earlier.