Xerox Corp. reported yesterday that net income for the third quarter of 1984 dropped 27 percent in large part because of a downturn in insurance operations, increased competition and strength of the U.S. dollar.

Also posting quarterly figures yesterday were Du Pont Co., Goodyear Tire & Rubber Corp., and General Foods.

Xerox net income for the third quarter this year was $81 million (72 cents per share), compared with $112 million ($1.05) for the same period in 1983, said David T. Kearns, president and chief executive officer.

Net income for the first nine months of 1984 was $303 million ($2.79), down 23 percent from $394 million ($3.78) last year.

"Market activity in our office equipment business continues well ahead of last year, but a number of factors continue to put pressure on our profitability," Kearns said.

Property and casualty insurance operations are a major contributor to the lower earnings, he said, but "encouraging signs of improvement" were shown in the third quarter.

Kearns also cited lower worldwide copier and duplicator prices, the strength of the U.S. dollar and lower gross margins from rental of equipment for lower earnings.

Revenues from Xerox Reprographics and Information Systems totaled $2.146 billion in the third quarter this year, up 6 percent from $2.032 billion a year ago. However, third-quarter income from those operations was $70 million, a decline of 7 percent from $76 million in the third quarter of 1983.

Revenues from Reprographics and Information Systems for the first nine months of 1984 were $6.419 billion, an increase of 3 percent from $6.205 billion last year. Income was $267 million, up 3 percent from $260 million for the first nine months in 1983.

Net additions to the number of Xerox copiers and duplicators leased and sold worldwide increased to record levels in the third quarter and for the first nine months this year.

The number of copies made by customers on leased and sold equipment was up 11 percent in both the latest quarter and in the first three quarters, compared with the same periods in 1983, Kearns said.

Du Pont said its profit, hurt by increasing foreign competition in its fibers business and a summer slowdown in other operations, rose only slightly in the third quarter from a year earlier.

The petroleum, coal and chemical products company reported profit of $315 million ($1.31) in the three months ended Sept. 30, compared with $312 million ($1.30) a year earlier. Revenue fell slighlty to $8.7 billion from $8.8 billion in the third quarter of 1983.

For the first nine months of 1984, the company said profit rose 43 percent to $1.1 billion ($4.67) from $786 million ($3.28) in the first nine months of 1983. Revenue rose to $27.1 billion from $26.3 billion in the first nine months of last year.

Du Pont Chairman Edward G. Jefferson said: "Results in July and August reflected the usual summer slowdown in many areas of our business, together with adverse effects on our fibers business of rapidly growing imports of synthetic fibers, fabrics and apparel."

Jefferson said that September demand for most product lines "improved to the levels achieved in the first half of the year. Margins for refined petroleum products have remained weak."

He added the growth of the economy is moderating and "many markets are increasingly affected by the overvaluation of the U.S. dollar against foreign currencies."

"The currency misalignment not only affects our export competitiveness, but is also resulting in higher imports of many products," Jefferson said.

Goodyear said its third-quarter profit edged up 4 percent from a year earlier on a 3 percent sales gain.

The earnings increase was paced by a 6.6 percent gain in overseas earnings; income from U.S. operations fell 3.5 percent, Goodyear said.

Still, Goodyear expects to post record worldwide earnings for all of 1984, surpassing the previous high of $368.4 million in 1981, Chairman Robert E. Mercer said in a statement.

In the third quarter, worldwide net income rose to $85.3 million (80 cents), from $82.2 million (83 cents) a year earlier. Third-quarter sales rose to $2.53 billion from $2.46 billion.

The modest profit increase produced a decline in earnings per share because the average number of common shares outstanding climbed to 106.3 million from 99.5 million a year earlier.

Sales of tires and other auto products to vehicle manufacturers "continued at a vigorous pace" in the latest quarter, but "competition in the domestic replacement tire market intensified," Mercer said.

Despite the declines in U.S. earnings, Goodyear said it was "experiencing the highest domestic demand for original equipment tires since 1979, and other automotive-related operations such as Motor Wheel, a producer of wheels and rims, also are experiencing an upsurge."

For the first nine months of 1984, Goodyear's profit jumped 87 percent to $305.7 million ($2.88) from $163.5 million ($1.65) in the comparable 1983 period.

Nine-month sales gained 7.4 percent to $7.68 billion from $7.15 billion.

General Foods, citing slowed sales of powdered beverages, the strong U.S. dollar and lower earnings from its Oscar Mayer meats subsidiary, reported a decrease in its second-quarter net income for fiscal 1985.

For the 13-week period ended Sept. 29, net income declined to $58.8 million ($1.20) compared with $73.4 million ($1.41) in the fiscal 1984 second quarter. Despite the strength of the dollar and the absence of revenues from its Gaines Pet Foods operation, which GF sold last June, revenue rose to $2.3 billion from $2.1 billion last year.

For the first six months of 1984 net income from operations amounted to $131.3 million ($2.61), compared with $133.2 million ($2.56).

The current period excludes a $98.3 million gain from the Gaines sale and restructuring of certain international operations. With these inclusions, net income totaled $170.3 million ($3.37). Revenue for the six-month period rose to $4.5 billion from $4.3 billion in the first half of fiscal 1984.