Exxon Corp. and Sun Co., the nation's largest and 10th-largest oil companies, each posted significantly higher first-quarter earnings yesterday. Shell Oil Co., the seventh-largest, said its earnings fell.

Also yesterday, Procter & Gamble Co. said its fiscal third-quarter profit rose 4.4 percent over a year ago on a 21.8 percent jump in sales.

Financial Corp. of America said first-quarter earnings of $49.1 million ($1.21 a share) compared with a loss of $38.1 million ($1.17) during the same period last year.

Times Mirror Co. reported that first quarter earnings fell 21.9 percent to $34.5 million (53 cents a share), from $44.2 million (64 cents) during the same period in 1985.

Exxon said its net profit totaled $1.7 billion ($2.35 a share), up 31 percent over the same period in 1985, when the company reported earnings of $1.3 billion ($1.71).

*Exxon said its revenue in the first quarter was $22.2 billion, down from $23.3 billion a year earlier.

C. C. Garvin Jr., Exxon's chairman, said that compared with 1985's first-quarter earnings, the latest performance was sharply reduced by the period's unprecedented decline in crude oil prices, accompanied by a continued weakening of the dollar.

During the latest quarter, the company's U.S. and foreign production income totaled $1.2 billion, vs. $1.3 billion a year ago.

*Shell Oil said its quarterly earnings were $276 million, down 7 percent from the year-ago first-quarter income of $296 million.

The company is wholly owned by the Royal Dutch-Shell Group of Companies, based in both London and the Hague, the Netherlands.

Revenue totaled $4.666 billion in the first quarter, vs. $4.743 a year earlier, the company said.

*Sun Oil posted a 20 percent increase -- to $146 million ($1.34 a share), from $127 million ($1.12) in the first quarter last year.

Sun's first-quarter revenue totaled $3.4 billion, which was down from $3.7 billion in the same period a year ago.

Procter & Gamble said growth in net earnings was hurt by an unusually low tax rate in the same quarter a year ago, the consumer products company said. Sales soared because the company included the sales of Richardson-Vicks Inc., which it recently acquired.

*P&G said its net earnings for the three months ended March 31 came to $167 million (98 cents a share), compared with $160 million (96 cents) in the same period a year earlier.

Sales for the quarter totaled $4.08 billion vs $3.35 billion.

For the first nine months of its year, net earnings rose 11.9 percent to $582 million ($3.46), from $520 million ($3.11) last year.

Nine-month sales rose 13.1 percent to $11.55 billion from $10.2 billion.

*Financial Corp. of America, which operates the American Savings and Loan Association, attributed the improvement to a "continuing favorable interest rate environment" that has led to an $85.5 million gain from the sale of loans and mortgage-backed securities.

Although FCA reported a $7.2 million pre-tax operating loss for the quarter, Chairman and Chief Executive Officer William Popejoy said the gains from loan sales have been used primarily for reinvestments.

The company also reported "stubborn problems" with its loan portfolio.

*Times Mirror Co., the Los Angeles-based communications company, which owns the Los Angeles Times newspaper as well as other broadcasting and publishing businesses, attributed its decline in part to economic conditions in Dallas and Denver, where it publishes the Dallas Times Herald and the Denver Post, and to improvement costs at those newspapers.

Although the company said its Los Angeles Times newspaper operations enjoyed an improved first quarter, earnings at its Newsday newspaper in Long Island, N.Y., were affected by start-up costs associated with the introduction of a New York City edition.