American Motors Corp. yesterday reported a net loss of $30.9 million for the third quarter, nearly double the $16.8 million net loss the automaker posted in the same period last year.

In other major earnings reports yesterday:

* E.I. du Pont de Nemours & Co. said that third-quarter net income was $195 million -- a 29 percent drop from a combined net income of $273 million for Du Pont and Conoco for the same quarter last year.

* Aetna Life & Casualty Co., a leading insurance and financial services concern, said that third-quarter net income fell 4.1 percent from a year earlier.

American Motor's third-quarter sales were $754 million, up from $648 million in the year-earlier period.

AMC reported a nine-month loss of $150.6 million compared with a loss of $89.4 million for the same period last year. Sales were $2 billion, unchanged from 1981 nine-month results.

"The general economic downturn and a resulting failure of the domestic industry to achieve higher sales volumes persisted through the third quarter of 1982," Paul Tippett, AMC chairman and chief executive officer, said.

Tippett also cited the effect of the further devaluation of the Mexican peso on international sales.

E.I. du Pont de Nemours & Co. earnings per share were 82 cents compared with 1981 earnings per share of $1.15. Sales of $8.2 billion were 1 percent below the $8.3 billion for the third quarter of 1981.

Nine-month net income was $692 million ($2.91 a share), 17 percent less than the combined net income of $835 million ($3.53) for the same time in 1981. Sales of $25 billion were up 2 percent from $24.6 billion in 1981.

Petroleum refining, marketing and transportation earnings improved by 44 percent. Petroleum exploration and production profit gained 23 percent, mainly on improved output in the North Sea and Libya. Coal and uranium earnings improved sharply.

On the other hand, Dupont's traditional chemical and manufacturing businesses declined. Physical volume in these businesses was down 12 percent in the third quarter from a year ago. The company's plant utilization rate dropped from 79 percent to 53 percent.

Aetna Life & Casualty Co. said that its earnings from operations rose to $150.1 million from $136 million in the third quarter of 1981. But in the latest quarter, Aetna suffered $18.4 million in losses stemming from securities transactions, which lowered third-quarter net income to $131.7 million ($1.37 a share).

In the third quarter of 1981, Aetna posted $1.3 million in capital gains, which pushed net income in that period to $137.3 million ($1.71).

Aetna's nine-month operating income rose to $372.4 million from $345.8 million in the comparable year-earlier period. In the latest period, however, a capital loss of $39.8 million lowered net income to $332.6 million ($3.86). A year earlier, Aetna reported $6.5 million in capital gains that lifted net income to $352.3 million ($4.38).

Third-quarter revenue edged up to $3.54 billion from $3.53 billion. Nine-month revenue also edged higher to $10.47 billion from $10.46 billion.

Higher earnings were reported in the employe benefits division, while declines were posted in the commercial insurance and personal financial security divisions.