Washington Post Co. profits more than doubled in the first nine months of 1982, and third-quarter earnings jumped to $8.5 million (60 cents a share) from a loss of $405,000 (3 cents) a year ago.

Revenues of Post Co. broadcasting and publishing businesses rose 6 percent for the quarter to $185.7 million from $175.3 million, and revenues for the 39 weeks ended Sept. 27 were up 8 percent to $576 million from $535.4 million.

Net income for the first nine months of 1982 rose to $30.2 million ($2.13) from $14.7 million ($1.04).

A Post representative said 1981 results were depressed by one-time losses on the sales of the Trenton (N.J.) Times, Inside Sports magazine and a television sales company, and by operating losses of those three properties. Excluding those businesses, earnings from continuing operations last year would have amounted to $4.3 million (30 cents) in the third quarter and $23.2 million ($1.65) for the nine months. The gains were produced by the newspaper division, whose revenues increased 19 percent and broadcasting stations, which were up 14 percent; Newsweek magazine's advertising pages and revenues continued to trail the previous year.

Among other local companies reporting third-quarter results yesterday, Pepco posted modest gains in earnings, while NS&T Bank's earnings dropped.

Potomac Electric Power Co. posted third-quarter profits of $52.9 million ($1.08), compared with $49.7 million ($1.04) in 1981, while operating revenue increased to $354.3 million from $334.8 million. For the nine-month period, Pepco earned $44.9 million ($1.82), up from $43.5 million ($1.71). Revenues grew to $850.4 million from $775.2 million. Pepco's directors changed the firm's dividend reinvestment plan by halting the 5 percent discount on the purchase of additional shares, making the common stock prices under the plan 100 percent of market price.

NS&T Bankshares, parent of D.C.-based NS&T Bank N.A., reported a drop in third-quarter earnings yesterday, from $1.5 million ($1.09) in 1981 to $1.3 million (98 cents). Renovations to NS&T's "nest" forced the company to rent office space, a company representative said, contributing to the decline in profits. For nine months, however, NS&T increased its earnings nearly 9 percent, to $4.4 million ($3.19) from $4 million ($2.92) last year. As of Sept. 30, the company had average assets of $513 million and deposits of $508 million.

Noland Co., a wholesaler of mechanical equipment to the building trades, posted third-quarter profits of $833,500 (34 cents) on sales of $63.4 million, compared with $1.3 million (55 cents) and sales of $70.6 million in 1981. For nine months, Noland earned $2.1 million (86 cents), compared with $3.1 million ($1.26) last year. Sales dropped to $183.2 million from $199.7 million.