Ford Motor Co. yesterday posted a record third-quarter profit of $380 million ($2.05 a share), bringing the Big Three auto makers to a combined $1 billion profit, just short of last year's record earnings.

Separately, No. 4 auto maker American Motors Corp. reported a profit of $2.5 million for the third quarter, but its per-share earnings were fractional. This compares to a loss of $9.1 million (14 cents a share) last year.

AMC's nine-month profits come to $12.3 million (4 cents). Last year, the auto maker lost $154 million ($2.25) in the first nine months.

Ford's statement followed Chrysler Corp.'s announcement of a record $261.6 million profit for the quarter, and General Motors Corp.'s earnings of $417 million.

This brought the three firms to a combined $1.058 billion. Last year, GM earned $737 million, Ford $333 million and Chrysler $100.2 million to set an industry record of $1.17 billion.

The slightly lower results this year are the result of reduced earnings by GM, which said strikes in the United States and Europe cost it $200 million during the quarter.

Ford's 1983 third-quarter profit, also a record, was $333 million ($1.83).

Ford Chairman Philip Caldwell credited the gain to improved car sales and market share, and the fact that consumers are buying more expensive products. Ford sales are up 29.1 percent so far this year, the most of any Big Three auto maker.

Ford also posted record nine-month earnings of $2.1 billion, compared with $1 billion in the first three quarters of 1983. The nine-month earnings exceed the $1.87 billion Ford earned in all of last year.

Ford had worldwide sales of $11.8 billion in the quarter, up 15 percent from last year's $10.2 billion. Ford's nine-month worldwide sales were $38.9 billion, compared with $32 billion last year.

Tenneco Inc. said its profits fell 20.8 percent in the third quarter from a year earlier and blamed the decline on disappointing results from its sales of refined petroleum products, such as gasoline, and a slump in its farm and construction equipment business.

Earnings fell to $126 million (76 cents), from $159 million ($1.05) a year earlier. Revenue slipped 1.6 percent, to $3.38 billion from $3.43 billion.

James Ketelsen, Tenneco's chairman, said rising oil and natural gas production was offset by falling oil prices.

For the nine months ended Sept. 30, profits rose 5.5 percent, to $501 million ($3.22), from $475 million ($3.12) a year earlier. Revenue was up 4.1 percent, at $11.05 billion from $10.61 billion.

Aetna Life & Casualty reported third-quarter operating earnings of $82 million (75 cents), including a one-time net benefit of $45 million. Earnings for the third quarter a year ago were $71 million (67 cents).

For the first nine months of 1984, operating earnings totaled $149 million ($1.32) share, compared with $257 million or ($2.43) for the same 1983 period.