Geico Corp. yesterday reported an increase of 32 percent in third-quarter earnings, and a slight increase in nine-month earnings to $98.1 million, from $97.6 million in 1983.

The company, which writes risk coverage for automobiles and homes, life insurance and reinsurance, reported a rise in net earnings for the quarter to $31.2 million ($1.61 a share), from $23.6 million ($1.09 for the same period last year.

Operating earnings for the third quarter rose from $25.1 million to $25.2 million, while operating earnings for the nine-month period were $75 million, compared with $70.5 million for the same nine-month period in 1983.

Washington Gas Light Co. reported a 40 percent increase in earnings for the 12 months ending Sept. to $40.4 million ($5.04 a share), from $28.8 million ($4.20) for the previous 12 months. The company reported revenue of $848.3 million for the latest period, up from $758.6 million.

The utility also reported drops in revenue to $85.9 million for the three months ending Sept. 30, from $90.7 million for the same period in 1983, and in profits to $5.6 million (76 cents) from $7 million ($1.12).

The company attributed the improvement for the 12-month period to 15 percent colder weather during the more recent 12-month period, cost control measures instituted by management, more customers and lower interest costs.

Meanwhile, Chesapeake Corp., a pulp and paper manufacturer based in West Point, Va., reported a tripling of earnings in its third quarter ended Oct. 7 to $7.9 million ($1.20 a share), from $2.5 million (39 cents) for the same quarter last year. Sales were $115.8 million, up 33 percent over sales of $87.2 million a year ago.

Nine-month profits increased to $16.7 million ($2.53) from $5.9 million (93 cents) for the same period last year. Sales were $273.9 million, up 30 percent over the same period last year.

The company attributed the increase in profits and sales to strong demand for its products and a $2 million reduction in 1984 taxes as a result of federal tax legislation allowing the cancellation of deferred taxes on export earnings between 1972 and 1984.

SCOPE Inc., a Reston-based electronics company specializing in the application of computer and signal-processing technology, reported that net income for the third quarter was $257,000 (21 cents a share), compared with $14,000 (1 cent) in the same period a year ago. Revenue increased approximately 44 percent to $4.1 million from $2.8 million.

Earnings for the nine months were $55,000 (5 cents), compared with $244,000 (20 cents) for the same period a year earlier. The company noted, however, that the figures for the first nine months of 1983 included the gain on the sale of securities and an extraordinary income tax credit. Sales were $10.3 million, compared with $7.5 million a year earlier.