Federal Realty Investment Trust, a Chevy Chase equity real estate firm, reported substantially higher profits for the third quarter and first nine months of the fiscal year ended Sept. 30.

The trust said net earnings for the quarter increased 18 percent, to $1.9 million (27 cents a share), from $1.6 million (26 cents) a year ago. Nine-month profits totaled $4.9 million (77 cents), compared with $4.2 million (72 cents) the year before.

Federal Realty said it earned $2.8 million from net cash flow operations in the third quarter, compared with $2.2 million the year before. Net cash flow from operations in the first nine months totaled $7.6 million, up from $5.6 million in 1983.

Potomac Electric Power Co. reported profits of $150.8 million ($2.92) for the 10 months ended Oct. 31, up from $123.4 million ($2.36) last year. Operating revenue for the 10 months equaled the $1 billion posted in the same period last year.

Meanwhile, Pepco said its profits for the 12 months ended Oct. 31 were $167.5 million ($3.21), while its operating revenue totaled $1.2 billion. For the comparable year-ago period, Pepco reported net income of $139.7 million ($2.66) on operating revenue of $1.2 billion.

Compucare Inc., a McLean firm that provides information processing and software for health-care institutions, said its third-quarter profits dropped to $275,000 (5 cents) from $596,000 (13 cents) last year. The company blamed the decline on a pretax operating loss of about $560,000 at its PCI division.

Net earnings for the nine months ended Sept. 30 were $1.8 million (33 cents), compared with $1.6 million (36 cents) for the 1983 period. Compucare said earnings per share declined because of an increase in the number of average shares outstanding.

Syscon Corp., a Washington computer systems engineering firm, said its revenue rose 19 percent in the third quarter, from $21.8 million to $26 million, while profits increased to $924,000 (30 cents) from $915,000 (31 cents). Net income for the first nine months of the current fiscal year totaled $2.9 million (93 cents), compared with $2.4 million (86 cents) last year. Revenue jumped 27 percent, from $60.9 million to $77.3 million.

Doughtie's Foods Inc. of Baltimore said its third-quarter profits rose $86,000 (9 cents) from $60,000 (6 cents) a year earlier. Sales for the quarter edged up slightly, to $13.5 million from $13.1 million.