Riggs National Bank, Washington's largest bank, yesterday reported a 27 percent increase in profits for the third quarter of this year.

Net income at the bank increased during the quarter from $5.1 million to $6.4 million, primarily because of increased level of earning assets and net interest yield on those assets, according to Riggs Chairman Vincent C. Burke Jr.

Earnings for the first time nine months also increased by 29.1 percent from $13.6 million ($4.50 a share) to $17.6 million ($5.88). For the first nine months the bank's deposit were up 12 percent from $1.8 billion last year to $2.1 bullion.

During the same time, loans increased 17.5 percent from $1.1 million to $1.3 million and assets rose 22.9 percent from $2.6 billion.

Results were for the bank and its whooly owned subsidiary Central Charge Service, Inc.

First Virginia Banks Inc. of Falls Church reported record quarterly earnings for the seventh straight quarter.

Net income for the third quarter rose 22 percent from $3 million (28 cents a share) to $3.7 million (35 cents a share). Net income for the first nine months increased by 20 percent from $8.6 million (81 cents a share) to $10.3 million (98 cents).

Bank Chairman Thomas K. Malone Jr. said that although earnings were high on a dollar basis, the increase over the previous quarter of this year was slight. Malone said that the corporation's increase in interest income was partly offset by a rise in interest costs as customers move their funds to higher-yielding savings instruments such as the six-month money market certificates.

Scientific Time Sharing Corp. of Bethesda reported its first quarter profits increased 36 percent to $336,000 (20 cents per share) from $213,000 (15 cents) in the same period a year ago.

The computer services and software company said its revenues for the quarter ended Aug. 31 increased on $4.5 million from $3.7 million.

Losses from European operations decreased slightly to $181,000 from $192,000, said President Daniel Dyer, and European business contributed to the company's increase in revenues.

Income per share increased less than total income, Dyer said, because Scientific Time Sharing has issued 250,000 additional shares of stock since last summer.

M. S. Ginn Co. reported profits fell by almost 10 percent last year, despite a 25 percent increase in sales.

Ginn blamed the downturn on inflation, the cost of relocating into new facillities and continued losses in its mail order division.

The Washington office supply chain reported sales of $47.3 million for the fiscal year ended June 30, up from $38.7 million. Profits fell to $932,000 ($1.28 per share) from $1.02 million ($1.42) last year.