American Airlines said yesterday its profits were up 11 percent for the third quarter, to $87.3 million from $78.6 million in the 1984 period.

Earnings reports were bleaker, however, for a number of other U.S. companies, as Monsanto Co., Inland Steel Co., Aluminum Co. of America, Zenith Electronics and Wang Laboratories all reported lower net income or losses.

American, which is based in Dallas, said its revenue for the quarter increased to $1.56 billion from $1.36 billion in the 1984 period.

For the first nine months of 1985, American's profits soared by 53 percent to $321.4 million from $210.3 million in 1984. Revenue for the same period increased to $4.64 billion from $4 billion last year.

*Monsanto said its net income for the third quarter dropped to $31 million (42 cents a share) from $78 million (97 cents) in the 1984 quarter. Consolidated net sales for the quarter, however, were higher: $1.76 billion, versus $1.60 billion for the year-earlier period.

For the first nine months, profits were $242 million ($3.14), down from $398 million ($4.87) in the corresponding 1984 period. Net sales for the first nine months were $4.99 billion compared with $5.13 billion in last year's period.

Monsanto's results for the third quarter include a portion of the quarterly results of G. D. Searle & Co., which the St. Louis-based chemical giant acquired earlier this year.

Richard Mahoney, Monsanto's president and chief executive officer, blamed the "disappointing" on weaknesses in the farm economy and the worldwide semiconductor materials markets, and the strong U.S. dollar.

*Alcoa of Pittsburgh said its third-quarter earnings declined 5 percent from last year, mainly because of low aluminum prices worldwide.

The company said its net earnings in the quarter ended Sept. 30 totaled $57.1 million (69 cents), compared with a profit of $60.1 million (74 cents) in the comparable 1984 period. Revenue slipped to $1.28 billion from $1.44 billion.

Alcoa's earnings for the first nine months dropped 61 percent to $104.3 million ($1.26), from $270.8 million ($3.32) for the like period last year. Revenue decreased to $4 billion from $4.4 billion.

*Inland, based in Chicago, blamed surging imports, slumping prices, lower steel shipments and higher operating costs for a third-quarter net loss of $51.9 million.

Inland, the nation's fourth-largest steel maker, reported a net loss of $39.6 million in the same quarter last year. Those results included unusual charges totaling $27.2 million.

The company said sales in the most recent quarter were $714.8 million, down 3 percent from $737.8 million in the 1984 period.

Inland had forecast a big loss for the 1985 third quarter and had eliminated the quarterly common stock dividend for the first time in 51 years. The company said a return to profitability in the fourth quarter was doubtful, but that there should be some improvement from the third quarter.

For the first nine months of 1985, Inland posted a $87.9 million loss, compared with a loss of $13.1 million in the like period last year.

*Zenith, based in Glenview, Ill., reported a third-quarter loss of $14.8 million, blaming severe price pressures in consumer electronics, depressed industry conditions for cable television decoders and lower component sales.

In the like quarter last year, Zenith reported earnings of nearly $21.9 million (99 cents). That includes a one-time tax adjustment.

Third-quarter sales were $361 million, down 17.2 percent from $436 million for last year's period.

For the first nine months, Zenith reported a loss of $11.3 million on sales of $1.14 billion. For the comparable 1984 period, Zenith had a $52.3 million profit.

*Wang said its profits fell sharply for the first quarter of fiscal 1986, even though revenue was up and the company set a new quarterly record for orders received.

Net earnings for the first quarter were $7 million (5 cents), down from $51.2 million (36 cents), in last year's opening quarter. Revenue for the quarter ended Sept. 30 increased to $560.9 million from $553.8 million a year earlier.