General Motors Corp. posted a $129.4 million third-quarter profit (41 cents a share) -- its largest for the period in four years -- but said the money came from its subsidiaries and cost-cutting, not from selling cars.

And Exxon Corp., the world's largest industrial concern, citing continued declines in demand for oil and chemical products, reported its profits edged down 0.5 percent from a year earlier as revenues fell 8 percent.

GM, which had reported a loss of $468 million ($1.59) in the third quarter of 1981, attributed the improvement to cost-cutting and manufacturing efficiencies. Good performances from GM's financing and insurance affiliates and net credit on income taxes because of low earnings also contributed to the gain, GM Chairman Roger B. Smith and President F. James McDonald said in a statement.

However, GM lost money in the production of cars and trucks, with an operating loss of $53.6 million for the quarter.

Net sales in the quarter were $14.3 billion, up 7 percent from $13.4 billion in the third quarter of 1981, the automaker said.

Net income for the first nine months of 1982 is $818 million ($2.64), compared with $27 million (76 cents) for the first nine months of 1981. GM sales during the first three quarters were down 6 percent from last year.

Factory sales worldwide in the quarter were 1.5 billion units, up 9.6 percent from 1.4 billion units in the quarter in 1981. Worldwide retail sales dropped to 1.5 billion units in this year's third quarter, off 12 percent from a year earlier, GM said.

Exxon Corp. reported its profits in the third quarter slipped to $1.07 billion ($1.23 a share), from $1.075 billion ($1.25) a year earlier. Revenue fell to $25.3 billion from $27.6 billion.

For the first nine months, Exxon said profit tumbled 29 percent to $3.2 billion ($3.68 a share), from $4.5 billion ($5.21), in the comparable 1981 period. Nine-month revenue climbed to $85.4 billion from $77.5 billion.

During the latest nine months, Exxon's operating earnings fell 34.1 percent to $2.64 billion.

Merrill Lynch & Co. Inc., citing gains stemming from the record stock market volume this summer, said yesterday its third-quarter profit nearly tripled from a year earlier and reached a high for any quarter in the company's history.

Merrill Lynch -- a New York-based diversified financial services concern and the nation's largest securities firm in terms of capital -- said its earnings also were boosted by gains in investment banking and in fixed-income transactions made for its own account.

Profit in the latest quarter soared to $100.3 million ($2.45 a share), from $35.1 million (88 cents) a year earlier. Revenue climbed 30 percent to $1.3 billion, also a quarterly record, from $1 billion.

The previous record for earnings came in the second quarter of 1981, when profits hit $67.7 million.

In the first nine months of this year, net income rose 12 percent to $166 million ($4.11), from $148 million ($3.77). Nine-month revenue gained 19 percent to $3.5 billion from $2.9 billion.

Commission revenues increased 31 percent to $284 million, Merrill Lynch said.

In other third-quarter reports yesterday:

Chicago-based McDonalds Corp. yesterday reported record net income, revenue and sales for both the third quarter and nine months ended Sept. 30.

Net income for the quarter was $86.89 million ($1.45 a share), up 15 percent from $75,82 million ($1.24) in the same quarter of 1981. For the nine months, net income totaled $232.1 million ($3.86), up 14 percent over $203.6 million ($3.35).

Systemwide sales for both periods were 10 percent higher than the comparable periods a year ago. For the three months, sales came to $2.10 billion against $1.90 billion; for the nine months, $5.86 billion against $5.34 billion

Texas Instruments' third quarter earnings increased by 36 percent over the same period a year ago, but company officials said they still may lay off more workers before the end of the year.

The company reported that its net income during the third quarter was $36.9 million ($1.57 a share), up 36 percent from $27.1 million ($1.15). Sales totaled $1.05 billion, a 1 percent gain over $1.04 billion.

For the first nine months of the year, company earnings totaled $101.5 million ($4.30), up 41 percent from $71.8 million ($3.06) in the first nine months of 1981. Sales for the period were $3.22 billion, up 2 percent from $3.16 billion.

Improved performance in TI's semiconductor and consumer electronics divisions, combined with continued strength in the government electronics business, "more than offset" the declines in other divisions, the company said.

American Broadcasting Co. increased its third quarter profit to $1.22 a share from $1.19 a year ago on a strong performance in television broadcasting.

Earnings from radio broadcasting and book publishing were down somewhat and losses of ABC Video Enterprises, the cable television program services, widened, Chairman Leonard Goldenson reported.

Net income for the quarter rose to $35.39 million on sales of $607.11 million from $34.09 million a year ago on sales of $553.56 million.

Nine months' profit rose to $118.96 million ($4.12) on sales of $1.905 billion, compared with $102.48 million ($3.60) a year ago on sales of $1.707 billion.

Boeing Co. yesterday reported lower earnings for both the third quarter and the first nine months of the year.

The Seattle-based aerospace company reported net earnings of $69 million (71 cents a share) in the July-August period, compared with $93 million (96 cents) in the third quarter of 1981. Sales in the 1982 period totaled $2.1 billion, down $158 million from the 1981 period.

For the first nine months of the year, earnings totaled $201 million ($2.08) on sales of $6.5 billion, compared with earnings of $377 million ($3.91) on sales of $7.4 billion in the comparable 1981 period.