The Federal Home Loan Mortgage Co. reported that its third-quarter earnings rose 9.8 percent, with an increase in its net interest margin more than making up for higher loan losses.

The company, known as Freddie Mac, buys mortgages from lenders and repackages them for the secondary market.

Freddie Mac also said yesterday that it is reviewing its portfolio of multifamily housing loans to determine if regular quarterly additions to its loss reserve are enough to keep up with the number of loans going bad in that market or whether it will have to make a special provision in the fourth quarter to cover those possible losses.

The Reston company's profit for the quarter was $123 million ($2.05 per share), up from $112 million ($1.86) in last year's quarter.

Its net interest margin -- essentially, its revenue -- was up $45 million in the quarter to $325 million. That margin includes income from fees, retained mortgages and the float between when a mortgage is bought and sold.

The company wrote off $124 million in bad loans in the quarter, including $81 million in apartment loans, compared with $49 million in the same period last year. Freddie Mac's loan-loss reserve as of the end of the quarter was $521 million. Year to date, the company said, its additions to that reserve exceed the number of loans written off by $47 million.

Columbia First Bank of Arlington, a federal savings bank, yesterday reported a $2.9 million loss for the year ended Sept. 30, compared with a profit of $27,000 (1 cent per share) earned in 1989.

For the fourth quarter, the thrift said it lost $5.3 million, compared with a loss of $438,000 in the same period last year.

Columbia First attributed the decline in fourth-quarter and year-end earnings to the downturn in the region's commercial real estate market, which forced the thrift to add $13.9 million to its provision for loan losses, the reserve that protects against possible loan defaults.

The increase in reserves offset a $3.9 million improvement in net interest income, $2.8 million recovered in the settlement of an insurance claim and litigation and $3.4 million in gains that resulted from a decline in the bank's debt.

Columbia First, which operates 27 banking offices in Virginia, Maryland and the District, had total assets of $2.1 billion at year-end. The thrift also said it exceeds all federal requirements for capital, the amount of cash that thrift owners must provide to protect the federal deposit insurance fund from losses.

Tempest Technologies Inc. of Reston, which sells computers equipped to meet military security standards, blamed a slow market for its loss in the quarter ended Sept. 30.

Tempest lost $88,000, compared with a profit of $104,000 (1 cent) in the same quarter last year.

Revenue was $1.974 million, almost unchanged from $1.975 million last year.

Tempest modifies computers to meet the federal government's Tempest standards for protection against electronic surveillance, as well as other military security standards.

President Stanley G. Peery said the loss was due to "the continued slowness of the Tempest marketplace and the government budget process."