Flow General Inc. of McLean yesterday reported a loss of $274,000 (3 cents a share) for the first quarter of its fiscal year, in contrast to a profit of $2.4 million (28 cents) a year earlier.

Flow General officials said the slip into red ink was attributable to "lower than expected revenues which were not completely offset by lower than planned costs." Revenues for the McLean high-technology firm climbed to $38.2 million from $30.2 million, but the company said European operations, sales of instruments and service revenues all fell short of goals.

The quarterly loss was the second in a row for Flow General, but the loss of 3 cents a share in the previous quarter came after the company took a one-time write-off of $1.7 million that wiped out the quarter's operating profit. The loss for the three months ended Sept. 30 was attributable entirely to operations and the impact of the recession.

Flow General has suffered a series of setbacks in recent months, including a Securities and Exchange Commission charge over disclosure to stockholders, a private lawsuit accusing executives of insider trading in the firm's stock and federal criminal charges involving an Army computer contract.

Systematics General Corp. of Washington said it had reversed 1981 losses in both the third quarter and the first nine months of this year. The company, which makes electronically secure computer terminals and other equipment, said it earned $432,000 (30 cents a share) on sales of $3.7 million in the quarter compared with a loss of $874,000 on revenues of $2.7 million in the 1981 quarter. The quarterly gain includes a $271,220 profit from the sale of real estate. For the first nine months of the year, Systematics General earned $537,000 (38 cents) on sales of $10.7 million, compared with a loss of $1.2 million on revenues of $9.1 million in the period last year.

Scope Inc. said it earned a small profit in the third quarter despite a decrease in revenues. The Reston electronics company said it earned $133,000 (11 cents) in the quarter, compared with a $3.2 million loss a year ago. The profit was attributable to a $155,000 tax credit in the quarter. Sales decreased to $2.8 million from $3.4 million in the quarter, as the company's lines of electronic productivity-improving industrial products were buffeted by the recession. In the nine months ended Sept. 30, Scope said it lost $453,000 on sales of $8.4 million compared with a loss of $2.2 million for the first nine months of 1981 on sales of $9.1 million.

Another local company, Insituform East Inc., a Landover firm that operates a licensed pipe-rehabilitation business using plastic tubing liner, said it earned $346,803 (27 cents) in the first quarter ended Sept. 30, on sales of $1.7 million, compared with a profit of $113,648 (11 cents) on sales of $1.2 million a year ago.

Baltimore-based Allegheny Beverage Corp. posted record earnings for the third quarter and nine months.

The firm posted profits of $2.7 million (63 cents) on revenues of $112.3 million, compared with $2.2 million (52 cents) on revenues of $110.8 million in 1981.

For nine months, Allegheny Beverage earned $6.1 million ($1.42) on revenues of $332.8 million, compared with earnings of $5.8 million ($1.36) on revenues of $306.5 million last year.