Washington Gas Light Co. suffered increased losses during the normally unprofitable third quarter while earnings of the Washington Post Co. soaring 43 percent, the firms reported yesterday.

Coal and rail strikes, meanwhile, caused a loss of $505,000 in the recent quarter for Pargas Inc., of Waldorf.

The Washington Gas quarterly report for the July-September period listed a net loss of $6.2 million compared with a loss in the 1977 summer quarter of $5.6 million. Although revenues rose slightly to $44.2 million from $42.9 million, all expense categories also increased.

For a 12-month period ended Sept. 30, which provides a better glimpse of the utility's financial condition because it includes the high-volume heating season during the winter, WGL listed profits of $15.5 million ($2.89 a share), down substantially from $21.4 million ($4.25) in the previous 12-month period.

Sales increased $41.5 million to $355 million in the recent 12 months but expenses climbed $45 million to $325 million. In addition, the year-earlier period includes a one time gain of 32 cents a share from settlement of a lawsuit.

Without the nonrecurring item, WGL earnings are off $1.04 a share on an annual basis which the utility attributed to inflation and warmer weather in the more recent year, which reduced natural gas consumption.

The Washington-based utility, which serves the entire metropolitan area, recently received permission to increase rates for Virginia customers by 8.2 percent, effective Oct. 13 and adding $8 million to annual revenues. Petitions to raise rates in D.C. ($11 million, filed in July 1977) and Maryland ($3.6 million, filed Oct. 16) are pending.

Washington Post Co. third-quarter profits totaled $9.5 million ($1.18 a share) compared with $6.6 million (79 cents) in the 1977 period, with per-share earnings rising more rapidly because there are fewer shares outstanding, in the wake of company purchases of its own shares. Quarterly revenues rose 21 percent to $123 million.

For the first nine months of 1978, the Washington communications company reported profits of $33.8 million ($4.15 a share) compared with $22.5 million ($2.63), an increase of some 50 percent that included substantial one-time gains of $4.4 million (54 cents) from the sales of WTOP radio to Outlet Co., and real estate.

Without these gains, earnings for the first three quarters were up about 31 percent as sales jumped 20 percent to $371 million.A company statement said all its three divisions - newspapers, magazines and broadcasting - had higher sales and pre-tax profits in the recent quarter.

The Newsweek magazine and book division reported a 19 percent increse in revenues while newspaper revenues rose 18 percent from the same quarter last year (not counting the Everett, Wash., Herald, acquired last February). Broadcast revenues rose 20 percent.

Pargas Inc.'s third quarter loss of $505,609 contrasted with year-earlier profits of $671M648 (17 cents a share) as revenues declined from $30.7 million to $28.1 million. Nine-months earnings plunged by two-thirds to $1.6 million (41 cents) from $4.9 million ($1.31) and sales increased to $110.5 million from $105.6 million.

Chairman William Hill said the company's main business, sales of propane, experienced record earnings in the nine months but that coal mining operations operated at a loss. Propane profits in the first nine months actually equalled the entire year of 1977. he noted.

Coal mines were shut twice in the third quarter with strikes by coal workers and twice because of rail strikes. Three strip mines have been closed and deep mine production is being expanded, Hill said. Profitable operations are expected in the current quarter, he added.

Hill also announced the election of N. L. Langley as president, effective Jan. 1, to succeed A. F. Anton, who is retiring. Langley has been with Pargas since 1946, most recently as executive vice president. Directors also voted a quarterly divided of 26 1/2 cents a share, payable Nov. 29 to owners of Nov. 15.