Du Pont Co. said yesterday that its profit for the third quarter ended Sept. 30 rose 32 percent to $452 million ($1.87 a share) on revenue of $7.6 billion, from profit of $343 million ($1.42) on revenue of $6.4 billion for the same quarter last year.

Chairman Richard E. Heckert attributed the profit rise at the Wilmington, Del., diversified chemical manufacturing giant to strong performances by its fibers, agricultural and industrial chemical and coal sectors, and improved results from upstream petroleum operations.

Profit for the first nine months of the year rose slightly to $1.3 billion ($5.57) on revenue of $22.5 billion, from profit of $1.2 billion ($4.97) on revenue of $20.5 billion, last year.

The third quarter result included a gain of $31 million (13 cents) from the sale of a Texas chlorine and caustic soda production facility, while last year's third quarter included a gain from nonrecurring items of $21 million (9 cents).

In other earnings results: UNC Inc., an Annapolis professional services and engineering company, said profit rose 32 percent in the third quarter to $5 million (30 cents) on revenue of $127 million. A year ago, third-quarter profit was $3.8 million (25 cents) on revenue of $138 million. Profit for the first nine months was $14.7 million (92 cents) on revenue of $481 million, up 15 percent from profit of $12.8 million (70 cents) on revenue of $418 million for the same period a year ago.

USLICO Corp., the diversified Arlington life insurance and financial services company, said profit for the third quarter was $6.3 million (62 cents) on revenue of $133.2 million, nearly double last year's third-quarter profit of $3.2 million (33 cents) on revenue of $134.7 million. For the first nine months of the year, the company earned $29.3 million ($2.45) on revenue of $371.2 million, a 24 percent increase from last year's nine-month profit of $23.6 million ($1.97) on revenue of $396 million.

Preston Corp., a trucking company based in Preston, said its third-quarter profit declined 88 percent, to $253,853 (4 cents), from $2.1 million (36 cents) a year ago, because of competitive pressure on rates charged trucking customers. The Maryland company's revenue, however, rose 20 percent in the quarter to $130 million from $107.8 million a year ago.

For the first nine months of the year, Preston's earnings declined 80 percent, to $1.3 million (23 cents) from $6.7 million ($1.16) a year earlier. Revenue in the nine months rose 18 percent, to $370.9 million from $313.7 million last year.

At First American Bank of Maryland, in Silver Spring, profit rose 19 percent in the third quarter to $2.2 million ($1.38), from profit of $1.8 million ($1.30) for the same period a year ago. For the nine months, profit totaled $5.6 million ($3.80), up 27 percent from $4.4 million ($3.39) for the first nine months of 1986. Loans increased 20 percent from the same period last year and deposits increased 17 percent, the bank said. Total assets as of Sept. 30 amounted to $970 million, an increase of 16 percent from last year.

Profit at Williams Industries, a Falls Church construction services firm, rose 123 percent in the year ended July 31. Williams earned $1.9 million (90 cents) on revenue of $88.3 million during the year, compared with a profit of $838,209 (45 cents) on revenue of $46.8 million a year ago.

Frank E. Williams Jr., president of the firm, attributed the rise in revenue to the acquisition of the John F. Beasley Construction Co. and Concrete Structures Inc. early in the fiscal year and the first full year of operation at the company's bridge-girder fabrication plant.

BDM International Inc., the McLean professional services company, said its profit increased 48 percent in the third quarter, to $4.5 million (38 cents) from $89.5 million (26 cents). Revenue declined in the quarter to $75.1 million from $89.5 million, the result, the company said, of a one-time gain that inflated last year's figure.

For the nine months, profit increased 51 percent, to $14.2 million ($1.19) from $9.4 million (85 cents). Revenue for the nine months declined slightly to $235.5 million from $240 million, again with last year's figures affected by the one-time gain.

The company said its third-quarter contract awards rose 50 percent to $234 million, while contract backlog at the end of the quarter stood at a record $550 million.