The Du Pont Co. reported a $282 million third-quarter profit yesterday, down 10 percent from the same period last year.

Separately yesterday, Bethlehem Steel Corp. directors voted to omit a quarterly dividend on common stock for the first time since 1938, and Sun Co., the nation's 10th-largest oil firm, reported a third-quarter loss of $160 million.

Du Pont said earnings per share were $1.17, compared with $1.31 for the same period in 1984. Sales were $8.5 billion, a 2 percent drop from $8.7 billion in the third quarter of 1984.

Profit for the first nine months of 1985 was $614 million ($2.53 per share), down 45 percent from $1.1 billion ($4.67) in the first nine months of 1984. Sales for the first nine months were $25.3 billion, a 6 percent drop from $27.1 billion for the first nine months last year.

The company said third-quarter profits were reduced by $88 million because of the closure of an ethylene production unit at the company's Chocolate Bayou plant in Alvin, Tex., and planned withdrawal of investment from the Syngas Co. and the shutdown of an associated methanol plant in Deer Park, Tex.

"Excluding the effect of abnormals in both years, third-quarter operating results were essentially unchanged from 1984," Du Pont Chairman Edward G. Jefferson said in a statement. "We have seen a significant improvement in our downstream petroleum business which has benefited from better refined product margins worldwide."

Offsetting the improvement were lower earnings from coal and some industrial and specialty businesses, he said.

Coal earnings were down because of lower sales, and the polymer products division reported lower earnings because of competitive prices from imports and weak demand in certain markets, the company said.

Bethlehem Steel Corp. yesterday said the quarterly dividend on common stock was omitted because of losses reported in the third quarter and expected in the fourth.

Declining steel prices and flat demand were blamed for net losses of $77 million and $119 million for the third quarter and first nine months of 1985, on sales of $1.29 billion and $3.83 billion in the same periods, Bethlehem spokesman Gary Graham said.

The losses include a charge of $35 million resulting from the planned shutdown in Lebanon, Pa., of the remainder of Bethlehem's industrial fastener operations, he said.

Net losses for the third quarter and first nine months of 1984 were $17 million and $48 million respectively on sales of $1.36 billion and $4.20 billion, the company said.

Bethlehem, based in Bethlehem, Pa., had been paying a quarterly dividend of 10 cents per share of common stock and the next dividend would have been payable Dec. 10.

"In accordance with previously announced policies, future common stock dividends will be determined by the board of directors on the basis of attained results in the business outlook," Graham said.

The company's basic steel operations segment had operating losses of $13 million and $24 million for the third quarter and first nine months of 1985, compared with losses of $13 million and $28 million for the same periods in 1984, Graham said.

Sun Co. reported a third-quarter loss of $160 million because it took $275 million in after-tax charges for plans to sell off some domestic oil, gas and mineral assets.

Sun, based in Radnor, Pa., said its operating earnings -- before the charges -- declined 7 percent on reduced results at Suncor, its 75 percent owned Canadian subsidiary, and lower U.S. exploration and production income.

Sun had an operating profit of $115 million in the third quarter, compared with net earnings of $122 million ($1.08 a share) in the same period last year. Revenue slipped 3 percent to $3.33 billion from $3.43 billion.

Sun's after-tax charges of $275 million include about $215 million for the divestiture of certain domestic energy and mineral holdings that are not meeting the company's return requirements and development plans. The write-down of other assets is covered by the remaining $60 million in charges.