Flow General Inc. told its stockholders to expect the worst in the third quarter, and they got it: Flow lost $2.8 million, against earnings of $389,000 (5 cents a share) last year.

For the nine months ended March 31, Flow has lost $4.4 million compared with a profit of $5.1 million (61 cents) in 1982.

Revenue for the McLean company increased, but not as much as expected, officials said. Third-quarter sales were $40.6 million compared with $34.5 million in 1982, while sales climbed to $120.2 million through three quarters of 1983 from $95.3 million last year.

Among the things contributing to Flow's performance is its divestiture of Flow Sensor and settlement of a contract dispute, which account for $1.5 million of the pretax loss. Flow also is trying to sell off Worthington Diagnostic Systems Inc., but hasn't found a buyer yet.

Flow also said it is charging to operations "slightly more than $1 million" in connection with the resignation of former chairman Joseph E. Hall. Other obligations Flow still might have to Hall from employment agreements, such as bonuses, have not been determined, Flow said, but "such costs will be recorded if and when the company becomes obligated to pay them."

Flow is a diversified company involved in medical and biomedical supplies, equipment and instruments. Its last four quarters have been troubled, to the point where Flow has said it is considering selling or licensing the technology for producing Interferon, the potential cancer-treatment drug.

General Physics Corp. of Columbia, a professional and technical services firm, posted first-quarter earnings of $456,000 (12 cents a share) compared with $717,000 (21 cents) in 1982. Revenue increased to $8.1 million from $6.9 million.

TVI Energy Corp. said yesterday its first-quarter earnings represent the company's first profit since it was founded in 1977. TVI, which makes thermal targets for the military, earned $42,737 (1 cent a share) compared with a loss of $306,510 last year. Sales for the first quarter were $903,843 compared with $295,704 in 1982.

Two real estate investment trusts, American Realty Trust of Arlington and Washington Real Estate Investment Trust, posted improved quarterly figures yesterday.

WRIT had first-quarter profits of $1.2 million (25 cents a share) on total income of $3.7 million compared with $1 million (22 cents) on total income of $3.5 million last year.

ART had second-quarter earnings of $1.4 million (64 cents a share), a big turnaround from its loss of $218,258 last year. Revenue climbed to $5.5 million from $2.9 million. First-half earnings were $1.2 million (54 cents) on revenue of $8.4 million compared with earnings of $5.8 million ($2.62) on revenue of $13.2 million.