Three insurance companies affiliated with Government Employes Insurance Co. reported second-quarter earnings yesterday, and two of them had decreases.

Government Employees Life Insurance Co. earned in profits, $2 million (46 cents a share) in the second quarter compared with $1.9 million (43 cents) a year ago.

First-half earnings of the life insurance company were $4.2 million (93 cents) compared with $3.8 million (84 cents) a year ago. Sales of individual life insurance increased 8 percent during the second quarter, and first-half sales were unchanged from the first half of 1977, Government Life said.

Criterion Insurance Co. reported a drop in second-quarter earnings to $2 million ($1.35 a share) from $2.4 million ($1.66) a year ago.

The auto insurance firm reported a sharper drop in first-half earnings to $2.9 million ($1.97) from $4.4 million ($3.03) a year ago. The company attributed the decreases partly to rising costs for auto repairs and substantial losses on policies written in New York.

Government Employees Financial Corp. said second-quarter profits fell to $750,000 (41 cents a share) from $879,000 (49 cents) a year ago.

First-half earnings of the consumer finance firm decreased to $1.5 million (84 cents) from $1.8 million ($1.04) a year ago.

The Denver-based company said it set first-half records in credit volume, finance receivables and revenues. It attributed the drop in earnings to increases in borrowing costs and advertising expenses.

The Rouse Co., a Columbia real estate development and management firm, reported second-quarter earnings of $616,000 (5 cents a share) compared with $186,000 (one cent) a year ago. The 1978 quarterly profits included a gain of $270,000 from the sale of properties, and the 1977 figure included a gain of $172,000.

First-half earnings were down slightly, to $2.4 million (18 cents) from $2.8 million (21 cents) a year ago. Gains from the sale of properties were $970,000 for the first half of 1978 and $422,000 for the same period a year ago.

Revenues increased to $24.4 million in the second quarter from $20.2 million a year ago. For the first half, revenues were $40.8 million a year ago.

MCI Communications Corp. reported a slight drop in earnings for its first quarter ended June 30 to $851,000 (3 cents a share) from $876,000 (3 cents) a year ago. Revenues for the period were $20.6 million compared with $17.7 million a year ago.

Chairman William McGowan said about $1.5 million of increased expenses during the quarter reflected the costs of expanding the company's Execunet long-distance telephone service in anticipation of a favorable court ruling, which the company received in June, allowing it to offer the service despite objections from American Telephone & Telegraph Co.

C3 Inc. reported profits of $1.5 million on sales of $7.5 million for the year ended June 30. Profits for the previous year were $751,000 on sales of $4 million. C3, based in Reston, manufactures mini-computer products and systems.

Computer Data Systems Inc. of Bethesda reported earnings of $410,000 (74 cents a share) for the year ended June 30 compared with $154,000 (27 cents) a year ago. Revenues for the year were $6.3 million compared with $5.8 million a year ago. Computer Data is a professional data services firm that provides consulting and data-processing support to state and local governments and private firms.