Potomac Electric Power Co. yesterday reported earnings of $48.6 million for the three months ended Sept. 30, and Pepco's board of directors declared a 38-cents-a-share common stock dividend for the fourth quarter of 1980.

Earnings per share for the third quarter were $1.06 compared with earnings of 85 cents per share in the same quarter in 1979 when earnings were $41.2 million.

Pepco said that earnings for the nine months ended Sept. 30 were $83.5 million ($1.72) compared with $64.7 million ($1.35) in 1979.

The dividend announced yesterday is payable on Dec. 31 to shareholders of record Nov. 26.

Computer Data Systems Inc. said yesterday that net income for the three months ended Sept. 30 was $249,000 (44 cents a share) compared with $152,000 (28 cents) for the same period in 1979. Revenues increased to $4.1 million compared with the year-earlier period.

According to Board Chairman Clifford M. Kendall, growth in programing and processing services offered by the company accounted for the increase in revenues. Together with improvement in pretax profit margins, they accounted for the increase in operating profits, he said.

CACI of Arlington, a research, analysis and programing company, yesterday reported earnings of $436,544 (42 cents a share) for the three months ended Sept. 30 compared with $363,319 (35 cents) in the same period in 1979. Revenues rose to $11.2 million compared with $6.5 million earned in a similar three-month period in 1979.