Washington Gas Light Co. yesterday reported it lost $2 million more than last year during the third quarter of the year, a period in which the gas company traditionally loses money.

Washington Gas showed a net loss of $7.9 million for the three months ended Sept. 30, up from a $5.9 million loss in the same period a year ago, even though revenues increased to $51.6 million from $41.7 million.

After paying preferred stock dividends, the quarterly loss for common stockholders amounted to $8.6 million compared with $6.6 million last year.

Gas company officials said the main reason for this year's summer loss was larger was a $12 million increase in the cost of natural gas purchased during the quarter. Last year the company collected funds from gas suppliers during the quarter.

Avemco Corp., a Bethesda aircraft insurnace and financing company, said its profits for the third quarter ended Sept. 30 fell to $109,000 (5 cents) from $754,000 (30 cents) last year.

For the first nine months of the year, Avemco earned $1.8 million (72 cents) down from $2.4 million (97 cents) in the same period a year ago.

UNC Resources Inc. reported earnings of $15.2 million ($1.43 a share) for the quarter ended Sept. 30 but disclosed it would have shown a $3.2 million pretax loss for the period if it had not sold off its coal properties.

Sale of the coal holdings gave the Falls Church nuclear resources company an extraordinary gain of $28.6 million for the quarter. A year ago UNC earned $765,000 (7 cents) during its second quarter.

For the first half of the year, UNC reported net earnings of $11 million ($1.04), up from $6.9 million (64 cednts) in the same period last year.

Pargas Inc., an LPG distributor and coal mine operator based in Waldorf, said third-quarter profits fell more than 50 percent to just over $1 million (28 cents a share) from $2.1 million (58 cents) last year, and earnings for the first nine months of the year slid to just under $7 million ($1.91) from $7.7 million ($2.11). Revenues for the nine-month period increased to $165 million from $125 million.