The Three Sisters Bridge, a once locally famous proposed interstate highway project that was to span the Potomac River above Georgetown, officailly was killed for the third or fourth time yesterday. This time it looks unusually permanent.

Announcement of the death of a project that had long-since been regarded as abandoned by highway officials came from Mayor Walter E. Washington. The city of Washington, he said, has received approval from the U.S. Department of Transportation to transfer U.S. Money from the bridge and another less-famous highway project to the Metro construction fund.

The less-famous project was the K Street tunnel, which was going to carry lanes of automobiles across town. The total amount of federal money made available by killing the two projects is $392 million. About $344 million will go to Metro, the rest to other D.C. street and bridge improvements the mayor said.

The Three Sisters Bridge - so named because it would cross the Potomac at the point where three rocks known as the Three Sisters are located - was to connect a branch of Interstate 66 with an unbuilt District of Columbia freeway. The branch of I-66 also is now dead, as is the D.C. Freeway.

Construction on the Three Sisters actually began in 1970, and about $1.2 million in work was completed before a court order stopped it. Before that court order, there were several days of media events - people standing in front of bulldozers, chanting, and making speeches about the ecological disaster of highway projects.

Another popular rallying cry in the antihighway movement was that the Three Sisters and other now defunct D.C. roads would be "white men's roads through black men's bedrooms."

In 1972 the House passed a law to prohibit the courts from intervening in the matter, but the provision did not survive in conference with the Senate.

Over the years the Supreme Court and the White House also have been involved, but work has never resumed.

Much of the work that was completed was ruined rock and silt that swept down the Potomac with the floods of tropical storm Agnes in June, 1972.