Communications Satellite Corp. reported added revenues from three of its earth satellite systems increased earnings for the third quarter by 11.5 percent, to $8.9 million ($1.12 per share) from $8 million (81 cents).

Comsat's revenues for the quarter were up $3.8 million to $46.7 million from $42.9 million. Comsat credited the gains to increased revenues from its Intelsat, Marisat and Comstar operations.

The company's board yesterday declared the regular 50 cents per share dividend payable Dec. 11 to shareholders of record Nov. 10.

For the first nine months of the year, Comsat's earnings were down slightly, to $24.4 million from $25.6 million. The company noted that the 1977 number included a credit of $1.4 million because of changes in accounting policies. Those changes did not affect the quarterly comparisons.

Earnings per share for the three quarters increased to $3.06 from $2.44. While the accounting changes added 15 cents per share to the 1977 figure, a reduction in outstanding shares boosted the 1978 figures.

Comsat said its revenues for the nine months increased to $132.7 million from $126.9 million.

Potomac Electric Power Co. reported increased earnings for the three months ended Sept. 30, but the gains were not enough to pull profits for the first nine months of the year up to 1977's levels.

On revenues of $242 million, Pepco earned $35.5 million (82 cents per share) for the quarter up from $34.9 million (79 cents) the same period a year ago when revenues totaled $223 million.

For the nine months, Pepco earnings fell to $62.3 million ($1.37 per share) from $65.7 million ($1.42), despite a gain in revenues to $551 million from $513 million.

Pepco's board declared regular dividends on all its outstanding shares.