Potomac Electric Power Co. reported yesterday a modest increase in profits during the third quarter ended Sept. 30, a period that includes the Washington-based utility's normal peak business from air conditioner usage.

Total profits were $35.5 million compared with $35 million for the same period a year earlier and per-share earnings were 82 cents compared with 79 cents, a gain of less than 4 percent. Revenues for the July-September quarter jumped some $19 million to $243 million but rising expenses kept pre-tax earnings to $52.3 million - a gain of less than $600.000.

Because of declining profitability in earlier months of 1978, and the absence of a strong third-quarter gain, Pepco's earnings for the first nine months are down from the same period a year ago.

Nine-month profits totaled $52.3 million ($1.37 a share) compared with $65.7 million ($1.42) last year despite an increase in revenues to $552 million from $513 million. And, on a 12-month comparison basis, Pepco's recent profits were $1.77 a share compare with $1.88 in the pervious period.

Citing increased inflation in materials and labor costs, Pepco has been seeking an increase in electric rates in D.C. of $44.9 million on a annual basis. The D.C. Public Service Commission currently has that proposal under review.

Pepco's directors declared a regular quarterly dividend of 33 1/2 cents on common stock, payable Dec. 29 to owners of record Nov. 29. Payouts on preferred and preference stock also were approved.

Ethyl Corp., a Richmond chemicals manufacturer, reported record third-quarter profits of $23.2 million ($1.14 a share) compared with a $20.9 million ($1.01) in the same period a year ago, an increase of 11 percent. Sales rose 7 percent to a record $360 million.

For the first nine months of the year, Ethyl earned $62 million ($3.03) compared with $60 million ($2.89) as sales increased 8 percent to $1 billion.

Chairman F.D. Gottwald Jr. said a decline in shipments of lead antiknock compounds for use in gasoline was more than offset by improvements in most other operations.

Regional banking companies continue to report sharp proift gains during the recent quarter, with Equitable Bancorporation up 20 percent and Bank of Virginia Co. up 15.5 percent.

Equitable, based in Baltimore, reported yesterday that third-quarter operating profits were a record $3.5 million (97 cents a share) compared with $2.9 million (81 cents) a year ago. Nine-month earnings, not counting gains or losses from selling securities, rose 20 percent to a record $9.4 million ($2.61 compared with $7.9 million ($2.18).

Bank company profits have been strong because of higher interest income on a volume of consumer and business loans that has remained on the rise despite soaring rates. Equitable's loan volume in the past 12 months rose 20.5 percent to an average of $1.165 billion in the recent nine months.

Bank of Virginia Co.'s third-quarter earnings, before securities transactions, were $3.4 million (72 cents a share) compared with $3 million (64 cents) a year earlier. Nine-month operating profits were up stringly to $9.8 million ($2.02) from $8.7 million ($1.88).

Chairman Frederick Deane Jr. said his holding company's banks had outstanding loans of $1.232 billion, up some 15 percent from a year ago. Among other bank reports:

First American Bank of Virginia, third-quarter proifts of $1.8 million ($1.11) vs. $1.55 million (97 cents); nine-month earnings of $4.8 million ($3.02) vs/$4.2 million ($2.63).

Union First National of D.C., third-quarter profits of $1.47 million vs. $1.06 million; nine-month earnings of $3.96 million vs. $2.97 million, not counting securities transactions.

Fidelity American Bankshares, of Lynchburg, third-quarter profits of $1.9 million (72 cents) vs. $1.5 million (55 cents); nine-month earnings before securities transactions of $5.5 million ($2.01) vs. $4.2 million ($1.58).

National Bank of Fairfax, third-quarter profits of $504,728 ($1.52) vs. $396,648 ($1.19); nine-month earnings of $1.4 million ($4.13) vs. $905,133 ($2.71).

New Virginia Bancorp., of Springfield, nine-month earnings of $1.4 million ($1.19) vs. $989,688 (84 cents).