American Management Systems Inc., one of the hotter computer services companies in the area over the last year, said yesterday its profits rose 46 percent in the third quarter and more than tripled in the first nine months of 1990.

For Arlington-based AMS, the latest results continued a string of sharp profit gains after losses in 1989, when it was facing a downturn in its federal government work and spending more than it expected to develop new computer software packages that are now fueling its growth.

In the first nine months of the year, revenue has been rising in nearly all of its markets, including the federal sector, at a time when that area is seen as particularly weak.

"They really have come all the way back. They're hitting on almost all cylinders," said Stephen T. McLellan, a Merrill Lynch & Co. analyst in New York. "I think that'll probably continue."

The company earned $3.5 million (31 cents a share) in the third quarter, compared with $2.4 million (22 cents) a year earlier. Revenue rose almost 20 percent, to $67.5 million from $56.4 million.

AMS said it earned $6.9 million (57 cents) in the first nine months, up from $1.9 million (17 cents) in the same period last year. Revenue increased more than 15 percent in the first nine months, to $189 million from $164.1 million.

McClellan said AMS, which develops generic software packages and tailors them to the needs of its clients, had in three years widened its markets and became less dependent on federal work. It also is benefiting from the corporate trend toward farming out big data-processing and automation projects, a movement termed "outsourcing," he said.

Its greatest growth has come in software sales to telecommunications companies, including the biggest long-distance carriers and the so-called Baby Bell regional companies, and to financial services companies.

Lafarge Corp., the Reston-based building materials producer, said a construction industry slump in central Canada and the Northeast United States led to a 45 percent drop in its profit for the first nine months and an 11 percent decline in third-quarter earnings.

Lafarge earned $54.5 million ($1) in the third quarter, compared with $61.3 million ($1.15) in the 1989 third quarter. Revenue rose 7.5 percent, to $538.7 million from $501.1 million.

The company reported a profit of $43.3 million in the first nine months, compared with $79.6 million in the same period of 1989. Revenue increased 9.5 percent, to $1.21 billion from $1.1 billion.

Black & Decker Corp. reported profits in the third quarter and first nine months, reversing losses last year related to its acquisition of Connecticut-based Emhart Corp.

The Towson, Md.-based power tools and home appliances giant said it earned $18.2 million in the third quarter, compared with a $29 million loss in the same period a year ago. Revenue rose 21 percent, to $1.3 billion from $1 billion.

Profits totaled $44.3 million (73 cents) in the first nine months, compared with an $8.3 million loss in the same period of 1989. Revenue gained 42 percent, to $3.5 billion from $2.5 billion.

Black & Decker said its operating earnings rose because of better manufacturing efficiency, cost controls and cost savings after it finished integrating Emhart's operations into its own. The higher revenue reflects the inclusion of Planning Research Corp., which it acquired in buying Emhart.

Mid Atlantic Medical Services Inc., the Rockville-based company that operates such health maintenance organizations (HMOs) as the M.D. IPA Health Plan and Optimum Choice, said its profit rose 160 percent in the first nine months and 88 percent in the third quarter.

Mid Atlantic earned $5.1 million (37 cents) in the first nine months, compared with $1.96 million (15 cents) in the year-ago period. Revenue rose 81 percent, to $150.1 million from $83 million.

In the third quarter, the company earned $1.9 million (14 cents), up from $1 million (8 cents) in the 1989 third quarter. Revenue rose 91 percent, to $56.3 million from $29.5 million in the year-ago quarter.

General Kinetics Inc. of Rockville said its profit nearly tripled in the fiscal first quarter ended Aug. 31, thanks to sharply rising sales for its fax machines, which are designed to be secure from electronic eavesdropping and are in demand because of the Persian Gulf crisis.

The company said it earned $404,107 (40 cents), up 196 percent from year-ago first-quarter earnings of $136,513. Revenue jumped almost 43 percent, to $5.16 million from $3.62 million.

Besides rising sales of its secure fax machines, General Kinetics said cost cuts, operational improvements and stronger financial controls boosted its bottom line.

James Madison Ltd., a Washington-based holding company for banks in the District, Maryland and Virginia and other financial subsidiaries, said a rise in troubled loans and other factors led to a third quarter loss of $16 million.

In the 1989 third quarter, James Madison earned $786,000 (7 cents). It lost $14.9 million in the first nine months, compared with a $2.4 million (24 cents) profit in the same period a year ago.

The bank company said additions to its reserve against potential loan losses, a loss on other real estate it owns and write-offs from a restructuring of its subsidiaries also fueled the loss.