Three more WashingTon area men died from apparent heroin overdoses Thursday night and yesterday, bringing the death toll to seven in a three-day string of fatal overdoses that officials describe as the city's worst ever.

"We don't have any idea why they're dying," said Capt. James Nestor of the D.C. Police Department narcotics squad. "It's just puzzling. Nothing like this has ever happened here."

Police initially assumed that the deaths were due to a new influx of powerfully pure heroin, but Nestor said that tests of several different batches of heroin seized around the city in recent days did not show any drugs pure enough to kill.

Heroin overdose deaths have been increasing steadily in Washington in recent years. This week's string of fatalities, while it is the worst single rash of deaths officials can remember, is part of that trend.

Last year Washington led the nation in heroin deaths per capita, with a rate that had risen from 1.9 per 100,000 people in 1979 to 2.8 deaths per 100,000 in 1981.

The D.C. medical examiner's office reported that as of yesterday 88 users had died this year from overdoses of heroin alone and another three had died from overdoses of heroin to which another substance, such as the drug dilaudid, had been added.

As of Oct. 31 last year, the closest comparable period for which figures are available, 94 persons had died from heroin overdoses. Officials say that the city this year is likely to record the second highest annual death total since such record-keeping began in 1971.

"The number of deaths here has been a very dynamic process," said D.C. Chief Medical Examiner James Luke. The National Centers for Disease Control in Atlanta has been working with Luke's office on a study of the problem, and a CDC staff member described heroin deaths in Washington as an epidemic that started in early 1979.

"We have no reason to think it has subsided yet," said James Ruttenbur, an epidemiologist with CDC's Center for Environmental Health.

"We're trying to get closer to really understanding what's involved in the death process," Ruttenbur said, characterizing the study as a first of its kind. "Two people shoot up the same amount of heroin from the same bag and one dies and the other wouldn't. We can't explain that."

Much of the study has concentrated on the relation between alcohol use and heroin.

Preliminary findings have shown that 74 percent of the people who have died in Washington as a result of heroin also had high blood alcohol levels. About half of the victims had enough alcohol in their bodies to be declared legally intoxicated, the study found.

It was not until last year, said Luke, that Washington's heroin death toll became particularly alarming, reaching 115. That was almost twice the number of deaths the preceding year and nearly triple the number of heroin overdoses reported the year before.

D.C. police and federal drug enforcement agents blamed much of last year's unprecedented surge in heroin deaths in Washington on an especially potent street mix of heroin called Murder I.

Officials looked for evidence of a similar high-powered batch of heroin when this week's overdoses began to appear, but so far with no success.

"We're not any further off than we were before as to why these guys have all died. We're just waiting for information from the street," Nestor said, adding that the odds against seven people dying from overuse of "normal drugs" are great.

Police Chief Maurice Turner said it is possible that there were impurities in the heroin or in whatever substances the victims used to dilute the heroin that could have contributed to their deaths.

Luke said yesterday that he has confirmed that six of the seven deaths were caused by heroin overdoses. Tests on the seventh are still under way.

Police declined to release the names of the three most recent overdose victims. They said that one victim, a 29-year-old man from Maryland, was pronounced dead Thursday at 3:08 p.m. at D.C. General Hospital. Later, another victim, a 25-year-old Northwest Washington man, was found in the basement of 3362 18th Street NW and pronounced dead at 7:03 at Howard University Hospital.

The last known victim, a 36-year-old man, was found at his home in the 700 block of Gresham Place NW and pronounced dead at 1:45 a.m. yesterday at D.C. General, police said.

The string of deaths began Tuesday evening with the death of a Northeast Washington man who was found unconscious with a hypodermic needle sticking in his arm in an elementary school bathroom. By late that night, two more Northeast men had died from apparent heroin overdoses, and a fourth was added to the list of victims Wednesday.