Flow General Inc., a biotechnology company producing the potential cancer-fighting agent interferon for trials by the National Cancer Institute, yesterday reported an increase in net income and sales for the first quarter of fiscal 1981 and announced a 3-for-2 split of its common stock.

Net income rose to $1.39 million (33 cents a share) on sales of $22.68 million in the quarter ended Sept. 30 from $713,000 (21 cents) on sales of $16.68 million during the same quarter in fiscal 1980.

The stock split is payable Dec. 10 to stockholders of record Nov. 12. Based on 4.63 million shares outstanding as of Oct. 3, the stock split will increase the outstanding shares to 6.94 million.

Flow General, which has its headquarters in McLean, manufactures and sells biomedical equipment and biological products, specialized equipment for radio and television broadcasters and materials-testing equipment.

American Management Systems Inc., a computer systems and services firm, reported third-quarter net income rose 29.1 percent to $634,000 (42 cents a share) on revenues of $15.63 million from $491,000 (33 cents) on revenues of $13.169 million for the same quarter of 1979. The revenues increase was 18.7 percent.

Nine-month earnings showed a 41.3 percent gain to $1.79 million ($1.20) from $1.27 million (90 cents) as sales rose 24.2 percent to $43.62 million from $35.13 million.

AMS designed a computerized financial management system for the District of Columbia but has since parted ways with the District government in a dispute over the operations of the system.

Dominion Bankshares Corp., in its first financial report since its merger in September with Valley of Virginia Bankshares Inc., reported income before securities transactions of $5.6 million (69 cents a share) in the third quarter compared with $5.4 million (67 cents) in the same time period in 1979. Net income was $4.9 million compared with $5 million.

Income before securities transactions for the nine months ended Sept. 30 was $16.49 million ($2.03) compared with $16.46 million ($2.05) in 1979, and net income was $15.1 million, down from $15.7 million. The corporation's financial data was restated to reflect the merger using a pooling-of-interests method of accounting.

Company officials said that 1980 earnings were affected by sharply higher money costs.

Equitable Bancorporation reported third-quarter earnings before securities transactions of $2.7 million (68 cents a share) compared with $3.6 million (97 cents) in the same quarter in 1979. Nine-month earnings before securities transactions were $6.71 million ($1.69) compared with $10.56 million ($2.66) in the first three quarters of 1979.