Systematics General Corp., a Sterling, Va., company that provides engineering services to the federal government and computer manufacturers, said its net income for the third quarter was up 27 percent to $548,000 (20 cents a share), compared with $431,000 (16 cents) in the same quarter last year.

Sales grew to $8.4 million from July through September, compared with sales of $6 million during the third quarter of 1983.

For the first nine months of the fiscal year, net income rose to $1.3 million (49 cents) on sales of $22.5 million, from $1 million (37 cents) on sales of $14.1 million during the comparable period last year.

* BDM International Inc., a McLean professional and technical services firm, reported that its third quarter net income increased 26 percent to $2 million (41 cents), compared with $1.6 million (33 cents) in the same period of 1983.

Revenue rose 20 percent to $48.8 million in the third quarter, compared with $40.7 million during the same quarter last year.

For the first nine months, net income rose 31 percent to $5.7 million ($1.15) on revenue of $136.2 million, from $4.3 million (89 cents) on revenue of $109.5 million during the same period last year.

* Dynalectron Corp., a McLean firm that is involved primarily in industrial construction and electrical engineering, reported that its third quarter net earnings went up slightly to $3.019 million (32 cents), compared with $3.015 million (31 cents) last year.

Revenue grew to $165.6 million in the third quarter, compared to $146.9 million for the third quarter of 1983.

For the first nine months, net income dropped to $4.6 million (48 cents) on revenue of $457.8 million, compared to $5.5 million (60 cents) on revenue of $385.1 million for the same period last year.

The company said the depressed world oil market has reduced demand for the support services it provides for the petroleum and petrochemical industries.

* Isomet Corp., a Springfield company that manufactures laser accessory components and equipment, said its net income for the third quarter decreased 37 percent to $18,000 (1 cent), compared with $29,000 (2 cents) during the same quarter last year.

Revenue for the first nine months of 1984 dropped to $2.3 million, compared with revenue of $2.7 million for the same period last year.

The company reported a loss of $204,000 (13 cents) on revenue of $2.3 million during the first nine months, compared to a profit of $81,000 (5 cents) on revenue of $2.7 million during the same period last year.