With conglomerates, petroleum companies, metals, mining, forest products, tobacco and chemical businesses leading the way, profits of American corporations stayed suprisingly high in the third quarter of 1979.

According to the quarterly Business Week magazine profits survey published yesterday, the all-industry average increase in profits for the third quarter was 22 percent over the same period a year ago.

Economists surveyed by the publication said they expect after-tax profits to reach an annual rate of $150 billion in the quarter -- up 9 percent from the strong second quarter of this year.

However, there was decline in corporate profits margins -- a measure of business profitability before taxes are paid, calculated by dividing pre-tax profits by total revenues.

Profit margins at the nation's largest firms surveyed by Business Week fell to 5.5 percent from 5.9 percent in the previous quarter.

In part, Business Week said the fat profit reports were deceptive because of several factors. For one thing, inflation in the value of inventories under certain accounting methods was $40.8 billion in the quarter, accounting for more than 16 percent of the total pre-tax profits.

In addition, lower effective tax rates, tax credits from accounting changes in Britain and gains from foreign currency transactions also fattened the paper profits.

According to the survey, conglomerates posted the largest year-to-year gain, at 100 percent. Large increases were posted by Dart Industries, Fuqua Industries and Teledyne. LTV, barely profitable last year, was up 760 percent; Litton Industries, in the red a year ago, showed profits of $77 million.

As reported in detail over recent weeks, oil industry profits also rose sharply. Fuel companies encompassed by the Business Week data reported a 97 percent combined increase for the third quarter over the same period last year.

Metals and mining earnings rose 89 percent; paper and forest products earnings were up 60 percent; tobacco earnings increased 48 percent; and chemicals profits advanced 34 percent.

Not all of the news was good. The automobile industry, one of the most basic to America's economy, had a quarterly net loss of $73 million that reflects not only a record $461 million Chrysler Corp. red-ink entry but also sharp declines at General Motors and Ford, even after certain tax and accounting change benefits that prevent actual net losses.

Appliance industry profits were also non-existent, with Singer Co. posting a loss of $125 million. Airline profits fell by 74 percent, tire and rubber earnings were down 67 percent, and both real estate and trucking listed a 6 percent dip in profitability.

Among Washington region companies included in the Business Week survey, prepared by Standard & Poor's Compustat Services Inc., Martin Marietta was up 21 percent in the conglomerate sector with a profit margin of 10.2 percent; Marriott Corp. in the food and lodging sector, was up 27 percent with a margin of 6.3 percent; Washington Post Co. was down 16 percent with a margin of 5.8 percent and Giant Food was up 7 percent with a margin of 1.3 percent (food retailing margins normally are very low).

Regional railroads included were Southern, up 74 percent (11.1 percent margin), Norfolk & Western, 199 percent (10.8 percent), Chessie System was down 50 percent (4.9 percent) and Seaboard Coastline was up 689 percent (5.7 percent).