A $3.9 million write-off on the sale of its Central Charge credit card system slashed away more than half the third-quarter profits of Riggs National Corp., Washington's biggest banking company.

Riggs' earnings fell to $2.94 million for the quarter from $6.4 million last year, and earnings for the first nine months of the year were down almost 16 percent to $15.6 million from $18.5 million last year. Riggs did not report earnings per share for the quarter, but said nine-month profits per share slipped to $2.61 from $3.09.

In a statement, the $3.4-billion-asset bank holding company said the sale of Central Charge Service Inc. "served to eliminate a persistent cause of an inadequate return on investment at Riggs during the last several years." The bank called the sale "a prudent and necessary step to advance the interests of the bank and its stockholders, which inevitably will have a favorable impact on earnings of the corporation in the future."

Excluding the loss on the sale of Central Charge, Riggs income was up more than 5 percent for the nine months ended Sept. 30 because of an increase in assets and higher interest rates, the bank noted.

In other reports:

Virginia National Bankshares Inc. of Norfolk posted record third-quarter net income of $8.3 million ($1.13 a share) for the third quarter, up almost 10 percent from $7.6 million ($1.06) a year ago. Nine-month earnings also set a record at $23.6 million ($3.20), up from $21.3 million ($2.98). Virginia national's assets reached $3.7 mbillion as of the end of the quarter.

Citizens Bancorp of Riverdale reported nine-month net earnings of $12.2 million ($6.78 a share), up 20 percent from $10.1 million ($5.66) last year. Total assets climbed 12 percent to $826 million for the holding company, which owns Citizens Bank and Trust Co. and Kennedy Bank and Trust Co.

STSC Inc. of Rockville reported earnings fell to $114,000 (5 cents a share) from $151,000 (7 cents) and revenues slipped by $66,000 to $6.9 million for its first quarter ended Aug. 1. The computer information systems firm was acquired last month by Continental Telecom Inc.

Wapora Inc., a Chevy Chase environmental consulting firm, posted a loss of $119,000 on revenues of $1.6 million for the six months ended Sept. 26, in contrast to a profit of $18,000 (3 cents a share) on revenues of $3.5 million. President J. I. Bregman said Wapora has received new contracts recently and "may reach a breakeven point during the coming quarter."

Computer Data Systems Inc., a Bethesda data-processing and professional services firm, reported net income of $389,000 (26 cents a share) for its first quarter ended June 30, up 94 percent from $200,000 a year ago. Revenues more than doubled to $8.5 million from $4.1 million.