General Motors Corp. posted third-quarter profits of $416.8 million ($1.31 a share), down from last year's record third-quarter earnings of $736.9 million ($2.33).

GM said that production losses caused by the United Auto Workers' mid-September strike at 17 sites in the United States and the early-July strike by the German metalworkers' union cost the company $200 million.

Other adverse factors in the quarter were continuing increases in the cost of labor and materials, 1985-model start-up costs and lower income from GM's financing subsidiary.

Meanwhile, Uniroyal Inc. said that its profits rose 82.5 percent in the third quarter from a year earlier, as gains in the chemical, rubber and plastic materials division offset declining earnings from tires and other products.

GM Chairman Roger Smith and President F. James McDonald noted in a statement that the giant auto maker's "outlook for the rest of the year is now clouded by a strike against GM Canada that affects other North American operations as well." Analysts estimate that the GM Canada strike, now a week old, could cost the parent auto maker at least another $200 million.

"With a prompt resolution of the Canadian negotiations, we are still optimistic about GM's prospects in the fourth quarter and over the long term," the executives said.

They also said that the acquisition during the quarter of Electronic Data Systems Corp., a giant Dallas-based data-processing firm, should be seen as a sign of GM's interest in improving its competitive position.

GM's dollar sales in the third quarter rose to $18.5 billion from $17.6 billion in the quarter a year ago. However, its vehicle sales were 1.79 million, down from 1.80 million the year before.

GM did post record nine-month net income of $3.64 billion ($11.51) this year compared with $2.43 billion ($7.73) in the first nine months of 1983.

So far this year, the auto maker has had dollar sales of $63 billion compared with $53.7 billion in the first three quarters of 1984.

Uniroyal Inc., a manufacturer of rubber, chemical and plastic products, said yesterday that its third-quarter earnings rose to $25 million (70 cents a share) from $13.7 million (38 cents) a year earlier. Revenue rose to $529 million from $509 million a year earlier.

Nine-month profits rose to $72.3 million ($2.03) from $42.5 million ($1.30) a year earlier. An extraordinary gain of $5.6 million (19 cents) from an exchange of common stock for debt boosted earnings in the latest nine months, Uniroyal said. Nine-month revenue rose to $1.68 billion from $1.5 billion a year earlier.

The company said the third-quarter improvement came largely from gains by the chemical, rubber and plastic-materials segment, which more than doubled its operating profits to $37 million from $16 million a year earlier.