With the death of The Washington Star now only three days away, another of the country's once-powerful evening newspapers faced extinction yesterday as the owners of the Philadelphia Bulletin said they would close it Aug. 16 unless its workers accept $6.3 million in wage and benefit reductions.

The Charter Co. of Jacksonville, Fla., owners of the 134-year-old Bulletin since April, 1980, said the paper was losing money at the rate of about $20 million a year -- the same figure that led Time Inc. to give up on the Star -- and can be saved only if the employes accept Charter's ultimatum.

Though its circulation of 406,000 makes it one of the largest newspapers in the country, the Bulletin has struggled for years against aggressive competition from the morning Philadelphia Inquirer and several other daily newspapers in the Delaware Valley and southern New Jersey. If it does close, it will follow the Star into the graveyard of prominent big-city afternoon papers that were snuffed out by changing living styles and economic conditions.

Prospects for saving the Star, already extremely slim, dwindled still further yesterday after a meeting between Mayor Marion Barry and top executives of Time, Inc.

"It's bleak, it's grim," Barry told reporters at the Hay-Adams Hotel after he met with Time president Richard Munro and Star publisher James R. Shepley. Barry said he and R. Robert Linowes, former president of the Greater Washington Board of Trade, who had been trying to save the Star through an employe-investor partnership, had "seen the numbers" and were convinced that Time "can't continue to lose millions of dollars in cash."

Friday's Star is scheduled to be the last in the paper's 128-year history, a history in which the Star, like the Philadelphia Bulletin, was once the dominant journalistic force in the community. Asked if Time would extend the deadline, Barry said, "Absolutely not." Because of the continued financial drain on Time, he said, "Aug. 7 is the day. It appears we will be a one-newspaper town."

Barry and Linowes wrote off their locally based effort to save the Star. It remains possible that some last-minute buyer will appear to take the paper off Time's hands and keep it going, but the Star itself is treating the decision to close down as final.

Termination notices were delivered to most of the 1,427 employes on Friday, Star workers said.

The Star has been carrying a full-page advertisement headed "Attention Washington area employers!" asking for jobs for Star employes. "Dedicated and skilled Star employes in dozens of job categories are seeking positions in and out of the Washington market," it said. Another ad, a notice to Star subscribers about final payment for their subscriptions, said flatly that the Star "will cease publication on Friday, Aug. 7."

President Reagan is scheduled to pay a farewell visit to the Star today.

Though discussions continued between Time Inc. and possible buyers, neither Time officials nor Star employes held out much hope that publication will continue.

For the past decade the Star competed only with The Washington Post. The Bulletin has had to compete with a prosperous morning paper, the Inquirer, with other papers in the city, and with dailies in nearby Camden, Wilmington, Atlantic City, Trenton and the Pennsylvania suburbs.

It has been losing money steadily for the past five years. Losses during the past 18 months totaled $23.7 million, and the rate of loss was accelerating, Charter said.

The death of the Bulletin would leave Philadelphia with three papers, but only one, the Inquirer, comparable in size and circulation. "To continue on as we have is folly," Bulletin publisher N. S. Hayden told employes and union leaders at a meeting where he made the announcement yesterday.

Charter said that only "cost-cutting concessions" from employes worth $6.3 million annually could save the paper. Of that total, $4.9 million would have to come in concessions from union workers, the company said.

It set a deadline of noon Aug. 16 for acceptance of the cutbacks by the seven unions representing workers at the Bulletin. Union leaders were scheduled to discuss the ultimatum at a meeting today.

"When you've got a company that's been losing a lot of money for an extended period of time," said Charter spokesman Tom McMorrow, "something has to happen."

"We are definitely not bluffing."

McMorrow said the form of the concessions, the exact combination of pay and benefit cuts, was subject to negotiation so long as the total saving was $6.3 million. The Bulletin employs about 1,900 people with an annual payroll of $42 million.

If the employes agree, Charter president J. P. Smith said in a statement, the company will include them in a profit-sharing plan under which 25 percent of the newspaper's pre-tax profits will go to them.

Smith also said that Charter would embark on revitalizing the paper if agreement is reached.

The paper has already laid off 125 employes and imposed other cost-cutting measures, including a pay freeze for non-union workers. But it closed 1980 $1.8 million "in the hole," Hayden said.

"It can have a happy ending," Hayden said, "and I'm hopeful and prayerful that it will. . . . The Charter Company is prepared to invest up to $30 million for capital and operating expenses. If all goes as planned, the Bulletin should be profitable in 1984. If we did not think we could achieve this, we would simply shut the doors today."