Flow General Inc., a McLean company that specializes in the manufacture of biomedical products and medical supplies, has reported a decline in third-quarter earnings.

The company said, however, that results for the three months ended March 31 were consistent with its expectations.

Profits for the quarter totaled $247,000 (3 cents a share), down from $417,000 (5 cents) in the comparable period a year earlier. Sales in the more recent quarter were up, however, from $35.9 million to $37.6 million.

For the nine months ended March 31, Flow said it had a loss of $2.3 million compared with a loss of $9 million at the end of the first three quarters last year. Sales increased, however, from $101.3 million to $106.7 million.

* Primark Corp., an energy and financial services company in McLean, also reported a slight earnings decline for the first quarter.

Profits for the quarter were $34.2 million ($3.58), compared with $37.2 million ($3.98) in the first quarter last year.

Net income for the 12-month period ended March 31 was $46.5 million ($4.91), down from $48.7 million ($5.35) a year earlier. Primark blamed the earnings decline primarily on weather conditions and variations in industrial production levels, which reduced utility sales.

The company is the holding corporation for Michigan Consolidated Gas Co.

* Classic Corp. of Jessup said net income for the third quarter increased $140,000, to $199,000 (10 cents). Sales were $9.6 million, up from $9.4 million in the comparable period a year earlier.

Net income for the nine months was $714,000 (37 cents), compared with $639,000 (38 cents). Total sales for the nine-month period, which ended March 31, were $29.1 million, up from $25.3 million.

* QuesTech Inc., a diversified high-tech holding company with subsidiaries specializing in electronic warfare technology, intelligence systems, aeronautical analysis and other research, reported a sharp decline in profits despite an increase in revenue for the first quarter ended March 31. Revenue increased 21 percent to $9 million compared with the same period last year, but profits dropped from $236,885 (19 cents per share) in the 1984 period to $80,000 (5 cents) this quarter.

Herbert W. Klotz, chairman of the McLean firm, blamed the drop in net income on a temporary shrinking of profit margins on certain defense contracts, continued substantial investment in a new subsidiary, and the reclassification of certain operating costs incurred over several years.