General Motors Corp. yesterday reported a $468 million loss for the third quarter, shocking even the pessimists in the auto industry, and confirming the damage done by the severely depressed sales and interest-rate concessions.

Although the third quarter is normally the least profitable for the industry because of model introductions, the loss announced yesterday is a particularly heavy setback for GM, which had been in the black for nine months following its $567 million loss in the third quarter a year ago.

GM Chairman Roger B. Smith and President F. James McDonald conceded that the long-awaited recovery in auto sales isn't just around the corner. "Until interest rates decline to more reasonable levels, we cannot look forward to a significant improvement in car and truck sales in the United States," their statement said.

They added that GM expects business activity to remain "sluggish" through the rest of this year, with a modest pickup beginning in 1982.

For the first nine months of the year, GM has earned $237 million (76 cents a share) compared with a loss of $824 million in the first three quarters of last year.

GM's worldwide third-quarter sales were $13.4 billion against sales of $12 billion in the 1980 third quarter. Nine-month sales were $47.1 billion compared with $41.5 billion in the same period of 1980.

GM's $468 million loss was for worldwide operations in the third quarter, and no breakdown for U.S. operations was provided. The loss would have been $250 million more if GM hadn't saved $190 million from foreign exchange transactions and $60 million from an accounting change in the handling of pension fund contributions, the company said.

GM said it sold 1.38 million cars and trucks in the third quarter worldwide compared with 1.4 million in the same quarter a year ago, and 1.7 million the year before.

U.S. car sales totaled 834,000 in the third quarter, and domestic truck sales came to 107,000. Another 120,000 vehicles were manufactured in Canada. The U.S. total for cars and trucks, 941,000, is virtually the same as the figure for the third quarter of 1980, the worst in GM's history.

GM was the second U.S. automaker to report third quarter results. Earlier, American Motors Corp. reported a $16.8 million deficit.

Both Ford Motor Co. and Chrysler Corp. are expected to produce losses of well over over $100 million each.

"You're going to see some lousy third- and fourth-quarter reports for the whole industry," Chrysler Vice President Wendell Larsen has predicted.

Larsen and other industry spokesmen have singled out high interest rates as the chief culprit in the industry's current distress. Financing charges of 15 percent and more have helped push the cost of new-car ownership well above $200 a month, creating a barrier that many potential car buyers refuse to cross. GM has responded by offering incentives that lower the effective interest rates on its small, front-wheel-drive cars to 12.9 percent.

Many private economists are not looking for a significant decline in interest rates for at least the remainder of the year.

GM's investment in plant and equipment rose by $3.8 billion in the third quarter, and Smith and McDonald said that, despite the new losses, GM would not back off its $40 billion, five-year plan to modernize its plants and redesign its car and truck lines. "We are confident that our product philosophy and direction are right for the times," the two GM executives said in a statement. But they added, "We recognize that confidence alone will not solve the sobering economic problems which confront us now."

Some auto industry analysts predicted that despite these intentions, GM would have to delay investments in future product lines and stretch out model development.

Beyond that, GM is likely to take a much harder negotiating position with suppliers and step up demands for wage concessions from the United Auto Workers, said analyst Arvid F. Jouppi.

GM used the third-quarter losses to send another signal to the UAW about a hard line in the next round of wage negotiations. Citing the difference in labor costs between the U.S. and Japanese auto industries, GM said "no manufacturer can prosper and no jobs are safe."

Jouppi said that the industry appears to be stuck with the rebates and interest-rate subsidies it adopted this summer to boost sales even though these concessions have heavily damaged profits.