C3 Inc. of Reston, which integrates mini- and microcomputer products and systems, reported a 70 percent increase in earnings for its third quarter, which ended Dec. 31. Profits totaled $742,000 (8 cents a share), compared with $436,000 (5 cents) for the same period a year earlier.

The increase accompanied a 52.7 percent increase in revenue for the third quarter to $19.7 million, up from $12.9 million a year earlier.

Despite the increases, however, C3 lowered its earnings estimates for its fiscal year ending March 31, forecasting revenue of about $70 million and earnings equal to 40 cents a share. By comparison, last year's profit of $4.3 million came to 47 cents a share.

Earlier, the company had expected fiscal 1984-85 earnings to be at least 60 cents a share. But the company said several events ate into profits, including the loss of several major proposed government contracts earlier in the year, the continuing costs of the Justice Department's criminal investigations into alleged improprieties involving older contracts, and planned adjustments in inventory and accounts receivable.

For the first nine months of the year, C3 earned $3.4 million (36 cents), down 27.7 percent from last year's $4.7 million (52 cents). Revenue, however, was up 17 percent to $53.7 million, from $45.9 million.

* Rising interest rates resulted in a 1984 loss of $57.4 million (87 cents) for the Federal National Mortgage Association (Fannie Mae). The loss compared with a profit of $75.5 million ($1.13) in 1983.

For the fourth quarter, the loss was $31.2 million (47 cents), compared with a net loss of $43.1 million (66 cents) for the same period a year earlier.

The company's negative interest margin -- the difference between what it receives in interest payments and what it has to pay in interest to support its lending -- increased to $78.5 million in the fourth quarter from $56 million in the third quarter. The negative interest margin for the year was $151.7 million, more than double the $61.5 million negative interest margin incurred during 1983.

Fannie Mae purchased $4.3 billion in mortgages during the fourth quarter, up from $3.2 billion in the third quarter. For the year, the company purchased a total of $16.7 billion, down from the $17.6 billion of mortgages bought in 1983.

* First American Bank announced a $7.3 million profit for 1984, a 30 percent increase over 1983's earnings of $5.6 million. For the fourth quarter, net income also increased 30 percent to $2.2 million, compared with $1.7 million for the same period a year earlier.

Assets, as of Dec. 31, totaled $1.1 billion, up 18 percent from the preceding year; loans increased by 33 percent to $663.1 million while deposits climbed 26 percent to $892.9 million.

* United Financial Banking Companies Inc. reported more than a 100 percent increase in profits for 1984 -- with net income totaling $713,165, up from $302,872 a year earlier. Earnings per share, however, remained relatively stable because the number of outstanding shares more than doubled in the interim. As a result, earnings per share were $1.22 in 1984, down from $1.25 a year before.

Deposits, as of Dec. 31, were $41.5 million -- more than double the $18.5 million held a year earlier.

* USLICO Corp., the holding company for United Services Life Insurance Co., reported net income of $7 million ($1.12) for its fourth quarter ended Dec. 31, a marginal drop from a year earlier, when aftertax profits totaled $7.03 million ($1.13). Net sales for the fourth quarter totaled $1.4 billion, up 73 percent from a year earlier, when sales totaled $808 million.

For the year, the company's net profits were $31.8 million ($5.12), up from $20.1 million ($3.22) in 1983. The company benefited from the new tax law that provided USLICO a net tax credit of $4.2 million for all of 1984 -- compared with a deferred tax liability of $5.5 million in 1983.