Flow General Inc., reported net income of $2.15 million (28 cents) in its third quarter ended March 31, a 122 percent increase from the $972,000 (17 cents) it earned in the third quarter of its 1980 fiscal year.

Flow General, which is based in McLean, said that sales rose 50 percent to $31.37 million in the third quarter from $20.88 million in 1980.

The company manufactures biological equipment, broadcasting equipment and devices to test materials. It also does material testing for clients and environmental and energy research, among other things.

For the nine months ended March 31, Flow General reported profits of $5.4 million (75 cents) on sales of $79.66 million in 1981, compared with $2.5 million (46 cents) on sales of $55.5 million in 1980.

First & Merchants Corp., the Richmond-based bank holding company, said it had net income of $3.86 million ($1.17 a share) during the first quarter of 1981, compared with $3.83 million (also $1.17). Before securities losses were figured in, the bank had income of $4.24 million ($1.29) compared with $4.1 million ($1.25) last year.

The Richmond, Fredericksburg and Potomac Railroad Co. said it had net income of $3.9 million ($10.99 a share), down from the $4.6 million ($12.99) it earned in the first quarter of 1980. During the first three months of the year the company had operating revenues of $13.6 million, compared with $12.6 million a year ago. The railroad reported that its net income was lower because the company had large tax credits in 1980 that it could not use this year.

Isomet Corp., of Springfield, reported first quarter net profits of $185,000 (12 cents a share) on sales of $1 million, compared with net earnings of $32,000 (2 cents) on sales of $532,000 during the first three months of 1980.

Commonwealth Natural Resources Inc. said that first quarter earnings fell 5 percent to $2.97 million ($2.56) from $3.12 million ($2.68) in 1980. During the same period revenues rose 9.5 percent, from $70.2 million to $76.8 million. The company said the revenue decline occurred because it had to record costs associated with supplemental gas production in the first quarter while it will not receive the revenues until later.

United Services Life Insurance reported new income of $3.15 million (54 cents a share), down from $3.34 million (59 cents) in the first three months of 1980. Revenues rose from $29.24 million in 1980 to $29.38 million.