Martin Marietta Corp. reported improved profits for the third quarter on the strength of its aerospace and construction operations.

Net earnings for the three months ended Sept. 30 amounted to $49.7 million (84 cents per share), compared with $45.5 million (77 cents) for the same three months in 1984, the company said. Nine-month net earnings totaled $199.5 million ($3.47), up from $142.7 million ($2.46) for continuing operations in the same period a year ago.

The nine-month figures included a previously announced one-time, pretax gain of $100 million from the sale of the company's Master Builders division. The company said it realized $22 million of pretax gains from asset sales in the comparable period of 1984.

*The Baltimore investment house of Legg Mason Inc. said increases in brokerage commissions and fees from its investment advisory businesses contributed to a nearly fivefold increase in profits in the second quarter.

Net earnings were $1.2 million (26 cents a share) for the three months ended Sept. 30, and $3 million (62 cents) for the fiscal year to date. These figures compared with $249,000 (5 cents) for the second quarter of 1984, and $683,000 (15 cents) for the first half of last year.

The company attributed the improved results partly to the addition of seven new branch offices in the past year. Legg Mason now has 38 offices.

*SYSCON Corp., a Washington-based high-tech firm that does significant work with the Pentagon, said profits grew 24 percent in the third quarter.

Net income for the third quarter totaled $1.1 million (37 cents per share), up from $924,000 (30 cents) in the same period a year ago. The company said the gain came on sales that grew 15 percent from $26.1 million in the 1984 third quarter to $29.9 million for the three months ended Aug. 31.

*Profits were essentially flat in the third quarter at Dynalectron Corp., a McLean-based technological services firm that is primarily involved in industrial construction and electrical engineering.

The company said net earnings for the third quarter totaled $3.2 million (33 cents per share), compared with $3.1 million (32 cents) for the same period in 1984. For the year to date, net earnings were $5 million (52 cents), compared with $4.9 million (49 cents) in the first nine months of 1984.

The company said its most profitable operation was its technical services segment, even though its international activities were down significantly. Dynalectron also said it had incurred severance and facility close-out costs as a result of recently announced decisions to divest certain unprofitable lines, although it said the impact of this on third-quarter figures was not significant.

*Vie de France, the Vienna, Va., baking and restaurant company, said profits dropped for the first quarter of its fiscal year as a result of management staff additions made in the previous year.

Net income totaled $1 million (10 cents per share) for the three months ended Sept. 21, compared with $1.4 million (13 cents) in the same period last year. The company said the additional management staff and training costs will help prepare Vie de France for future market expansion and reacceleration of growth.

*Universal Security Instruments Inc., of Owings Mill, Md., announced that income doubled in the company's second quarter, rising from $65,631 (2 cents per share) to $145,332 (4 cents).

Net earnings for the year to date totalled $215,568 (6 cents), as compared with a loss of $904,200 (24 cents) for the same period a year ago, the company said. Universal designs, manufactures and markets video products and security equipment, among other products.

*General Physics Corp. said it recorded earnings of $1.1 million (31 cents per share) in the third quarter, a record figure for the firm and 59 percent higher than the 721,000 (19 cents) in earnings enjoyed in the same period in 1984.

The company said the results reflected expanding revenue and earnings from the training, operations, engineering and maintenance services the company provides to the power industry. Revenue for the three months ended Sept. 30 totaled $17.5 million, compared with $9.6 million in the same period in 1984.