Government Technology Services Inc., a Chantilly firm that sells computers to the government, said it earned $2.7 million (30 cents a share) in its third quarter ended Sept. 30. In the same period of 1998, GTSI reported net income of $4 million (40 cents). For the first nine months of the year, GTSI earned $923,000 (10 cents), up from $618,000 (7 cents) in 1998.

Third-quarter revenue was $194.5 million, compared with $196 million in the third quarter of 1998. GTSI generated $468.8 million in revenue in the first three quarters of this year, up from $432 million in the first three quarters of 1998.

* GTS Duratek Inc., a Columbia company that sells radiation-protection products and services, earned $2.9 million (15 cents a share) on revenue of $44.8 million in the third quarter ended Sept. 30, compared with net income of $1.9 million (9 cents) on revenue of $39.4 million in the year-ago quarter.

For the first nine months of the year, the company earned $7.4 million (37 cents) on revenue of $125.4 million, compared with earning $1.2 million (7 cents) on revenue of $115.4 million in the same period in 1998.

* Halifax Corp. of Alexandria said its fiscal second-quarter earnings were $147,000 (7 cents a share), compared with a net loss of $2.5 million in the same quarter in 1998. Second-quarter revenue increased 27 percent, to $21.7 million.

Earlier this year, the company said it discovered a $15 million embezzlement scheme by a former employee, Mary Adams Collins in its Richmond-based subsidiary.

Halifax restated its earnings for 1997, 1998, and the first three quarters of 1999, which was the main reason for the earnings turnaround. Halifax stock closed yesterday at $6.75, up 12 1/2 cents.