Potomac Electric Power Co. yesterday reported a slight rise in third-quarter earnings to $49.73 million ($1.04 a share), compared with $48.59 million ($1.06) for the same quarter in 1980. Pepco's revenues for the three months rose to $334.83 million from $300.78 million.

Average shares outstanding for the quarter increased 1.5 million over last year's figure, accounting for the lower dollar-per-share earnings in the third quarter.

The Washington electric utility company reported nine-month profits of $86.43 million ($1.71) compared with $83.45 million ($1.72), as revenues for the three quarters rose sharply, from $647.59 million to $775.24 million.

Maryland National Corp. was among a number of bank firms that reported earnings increases, posting third-quarter profits of $9.95 million ($1.30) compared with $9.02 million ($1.26) for the same quarter last year.

For the first three quarters, the holding company that includes Maryland National Bank, the state's largest bank with assets of $3.8 billion, reported profits of $30.08 million ($3.94) compared with profits of $24.58 million ($1.26) for the same period last year.

Also in Maryland, Equitable Bancorporation reported third-quarter profits of $3.05 million (74 cents), a 13 percent increase over 1980 profits for the quarter of $2.69 million (65 cents). The results bring Equitable's nine-month profits to $8.31 million ($2.01), up from $6.71 million ($1.61) for the same period last year.

National Savings & Trust Co. reported a 16.8 percent increase in earnings for the third quarter, as profits rose to $1.55 million ($3.41 a share), compared with $1.33 million ($2.92) for the same quarter last year.

Profits of the D.C. bank for the first nine months of 1981 rose 17 percent to $4.09 million ($8.95), compared with $3.5 million ($7.66) for the 1980 period.

The Student Loan Marketing Association (Sallie Mae), reported a 66 percent increase in third-quarter profits, as the organization continued to improve its financial picture.

Sallie Mae's profits for the quarter rose to $5 million ($5.03) compared with 1980 third-quarter income of $3 million ($3.04). For the first nine months of the year, profits rose 92 percent to $12.6 million ($12.63) compared to profits of $6.6 million ($6.57) for the same period last year.

The association, a national secondary market for guaranteed student loans, reported 90 percent increase in assets, with loans rising 90 percent to $4.3 billion on Sept. 30, up from $2.3 billion on the same date last year.

F&M National Corp. reported third-quarter profits of $544,000 (33 cents a share) up from profits of $401,000 (24 cents). The Winchester, Va., company reported nine-month profits of $1.6 million (98 cents) compared with profits of $1.4 million (85 cents) for the same period last year.

Wapora Inc. reported a loss of $8,354 (one cent a share) for the first quarter of the company's fiscal year, compared with profits of $81,422 (14 cents) for the same period in 1980. Sales of the Chevy Chase environmental consulting firm dropped from $1.82 million last year to $1.56 million for the quarter this year.