C3 Inc. yesterday said its third-quarter profit jumped nearly five-fold, in part because of a $5.6 million one-time gain from the sale of a portion of the Herndon-based computer company's interest in Tempest Technologies Inc., formerly a wholly owned subsidiary.

C3 said its third-quarter profit was $7.9 million (79 cents per share), compared to $1.7 million (17 cents) for the same quarter last year.

The company's revenue rose 40 percent in the quarter to $35.3 million from last year's quarterly revenue of $25.3 million.

For the nine months ended Dec. 31, C3's profit jumped to $12.7 million ($1.27) from $2.2 million (22 cents) last year, again aided by the sale of Tempest stock.

Nine-month revenue rose 29 percent, to $84 million from $65.2 million.

Dominion Federal Savings and Loan Association of McLean said it lost $1.6 million (32 cents) in the third quarter, compared to a profit of $6.1 million ($1.21) a year ago.

The institution, which is appealing a potentially crippling $129 million judgment against it last year in a lawsuit by Penthouse International Ltd., said the quarterly loss was attributable to a "significant reduction" in demand for residential lending.

It said its residential mortgage closings dipped to $513 million in the nine months ended Dec. 31 from $754 million in the same period a year ago.

For the nine months, the company earned $3.1 million (62 cents), down from $15.4 million ($3.05) in the same period of 1986.

Total assets increased to $1.96 billion as of Dec. 31 from $1.73 billion a year earlier.