The Washington Post Co. yesterday reported a net loss of $440,000 in the third quarter ended Sept. 27--the first quarterly loss in recent history for the communications firm.

A company announcement attributed the loss primarily to the sale of two subsidiaries: the Trenton Times newspaper, sold last Friday to Washington financier Joe L. Allbritton after three unprofitable years, and a national television sales representation unit, sold early in the quarter.

Separately, Planning Research Corp. reported a 27 percent increase in profits for the quarter ended Sept. 30, while Pargas Inc. posted record profits in the first nine months of 1981.

The Post Co. loss in the quarter was in contrast with earnings for the year-earlier period of $2.5 million (18 cents a share) as revenues rose to $175.3 million from $152.5 million.

Not counting one-time writeoffs for units that were sold, taxes or the Post Co.'s share of earnings at affiliated businesses, operating profits of the company were up in the recent period to $5.4 million from $4.1 million. But the Trenton and TV unit sales resulted in a one-time loss before taxes of $4.3 million, which was charged against earnings in the quarter even though the Trenton sale formally took place at a later date.

If these events had not taken place, the Post Co. said, net income in the recent quarter would have been $2.6 million (19 cents a share), an increase of 4 percent from last year.

In addition, the company said the closing of The Washington Star in August has had a dramatic impact on The Post newspaper here: Circulation is up 25 percent daily and 16 percent on Sundays, which has led to higher operating expenses in production and distribution. Pre-tax operating earnings of The Post newspaper were trimmed $2.5 million as a result, although the company said "this adverse effect will not be as large in the fourth quarter."

In the wake of circulation gains, The Post has announced advertising rate increases that took effect Oct. 1 and a boost in newsstand prices of the publication, effective yesterday. In the company's magazine division, revenues rose 13 percent in the recent quarter and pretax profits were higher, as Newsweek ad pages increased and promotional costs of Inside Sports magazine were reduced. Broadcast division revenues rose 19 percent, but expenses associated with new TV productions caused a decline in pretax profits, the firm stated.

For the first three quarters of the year, the loss in the recent three months was a factor in lower profits of $14.3 million ($1.02 a share) compared with $18.9 million ($1.35) a year ago as revenues rose to $535 million from $474 million. One bright spot in the third quarter was the firm's share of affiliates' earnings, up to $1.5 million from $200,000 a year ago and reflecting improved operations at the Bear Island newsprint factory in Virginia.

Planning Research Corp., a professional services company based in Washington and McLean, reported profits for the first fiscal quarter of $1.86 million (27 cents a share) compared with $1.47 million (22 cents) a year earlier. Revenues rose to $79.1 million from $75.9 million.

President John Toups attributed the gains partly to the shutdown of unprofitable operations as well as implementation of a new planning program. Strong demand from the government for computer systems and services was a major growth factor, he said.

Pargas Inc. of Waldorf, a distributor of propane and coal mine owner, reported third-quarter profits of $1.74 million (48 cents a share) compared with $1 million (28 cents) a year ago as sales rose to $52 million from $38 million. Nine-month earnings were a record $7.75 million ($2.13) compared with $7 million ($1.91) as revenues rose to $201 million from $166 million.

President N. L. Langley said coal shipments and profits increased significantly while the sale of propane for use as motor fuel also added to overall volume of liquefied-petroleum gas distribution.

Government Employees Life Insurance Co. of Washington reported third-quarter operating profits of $2.1 million (47 cents a share) versus $2.13 million (48 cents) a year ago. Counting losses from capital transactions in both periods, net income fell to $2.06 million (46 cents) from $2.13 million (48 cents). Nine-month earnings from operations, not counting capital gains or losses, declined slightly to $6.05 million ($1.36) from $6.18 million ($1.39).

The Geico Corp. unit said recent results reflect higher policy terminations and benefit payments and expected start-up expenses for a new brokerage division. Individual life sales in the first nine months totaled $288 million, more than double the 1980 rate.

W. Bell & Co., of Rockville, a catalogue retail firm, said earnings in its first fiscal quarter ended Sept. 26 rose to $126,800 (10 cents a share) from $82,800 (7 cents) in the traditionally slow period.