Atlantic Richfield Co. said yesterday that its third-quarter earnings rose 22 percent from last year's level, Shell Oil Co. reported a 20 percent increase over the same period, and Tenneco Inc. said its profits were 24 percent higher.

Arco is the nation's seventh-largest oil company, Shell is No. 8, and Tenneco is No. 10. The only oil company among the 10 largest yet to report third-quarter results is Mobil, No. 2.

Atlantic Richfield's third-quarter earnings were $389.7 million ($1.57 a share) and came on sales of $5.6 billion amid increased oil production, improved refining and marketing results and rising petroleum prices. Last year's third-quarter earnings totaled $320.4 million ($1.30) on sales of $4.4 billion.

Nine-month earnings were $1.3 billion ($5.04) on sales of $16.8 billion against $822.9 million ($3.35) on sales of $11.6 billion a year earlier.

Arco said its earnings were helped by a $115 million net gain, after the sale of an interest in a Colorado oilshale venture to Exxon Corp. offset costs of its shutdown of a coppersmelting and refining operation in Montana.

The earnings figures surprised some Wall Street observers, who had expected them to be higher. At Merrill Lynch, Pierce, Fenner & Smith Inc., which had forecast earnings of $1.80 a share, analyst Constantine G. Fliakos said the report "clearly was disappointing."

While Atlantic Richfield's crude oil production from the North Slope of Alaska showed a 17 percent increase over 1979 levels, it said its pretax petroleum refining and marketing earnings slumped 68 percent to $35 million as rising oil prices prompted consumers to use less fuel.

Shell said, meanwhile, that its third-quarter profits were $353 million ($1.14 a share) on sales of $5.1 billion compared with 1979 third-quarter earnings of $293 million (95 cents) and sales of $3.8 billion.

Unlike Atlantic Richfield, Shell reported a 108 percent jump in profits from its oil-products operations to $81 million, as an improvement in supplies enabled the division to rebound from its poorest showing since 1973.

Shell said nine-month earnings were $1.1 billion ($3.67) on sales of $14.8 billion, up from $794 million ($2.59) and sales of $10.2 billion in the third quarter of 1979.

Despite a 5 percent drop in petroleum product sales in the third quarter, the Lundberg Letter said this week that Shell's improved supplies allowed the company to become the leading U.S. gasoline marketer in the first half of 1980.

The trade publication said Shell took a 7.62 percent share of the gasoline market in the first half against the 7.6 percent held by Exxon, last year's industry leader.

Tenneco earned $162 million ($1.25 a share) in the third quarter, up from $131 million ($1.20) a year ago, as sales rose to $3.068 billion from $2.789 billion.

Nine-month earnings were $526 million ($4.38) on sales of $9.535 billion compared with $407 million ($3.80) a year ago on sales of $7.94 billion.

Chairman James L. Ketelson said all of Tenneco's many divisions operated profitably and that four -- oil, shipbuilding, farm land management and packing -- had significant gains.