SAN FRANCISCO (AP) _ A flood of potent Mexican black tar heroin washed across San Francisco over the weekend, killing three users and sending dozens to hospitals with injuries ranging from broken bones to brain damage.

''Black tar heroin is pretty potent stuff anyway,'' said police Inspector Sam Osborne. He said the drug normally sold on the street is 5 to 9 percent pure. ''What we think is somebody forgot to cut it somewhere along the line.''

Between Friday and Monday morning, three people died in San Francisco and 54 others were treated for overdoses after they shot up the gooey black drug.

Some users were unconscious and unable to breathe when reached by paramedics who administered an antidote called Nalaxone. Overdose cases were reaching San Francisco General and a few private hospitals at the rate of one an hour, compared with the usual heroin overdose rate of one or two a day, authorities said.

The fatalities were said to be men aged 35, 41 and 43. The coroner's office was not releasing their names on Monday.

At the Haight-Ashbury Free Clinic, Medical Director Dr. Alex Stalcup said the latest overdoses were the worst since a ''similar wave of overdoses and deaths'' in March 1987 that involved black tar mixed with fentanyl, a pharmaceutical compound that can be 1,000 times more powerful than heroin. It is used by anesthesiologists and redirected illicitly.

Stalcup speculated that the same thing may have happened in the recent overdoses. Introduction of fentanyl into the drug scene several years ago resulted in a sudden rash of even more deaths, he said. Today it's often mixed with an adulterant and sold as China White, he added.

Black tar has become the most commonly used heroin by San Francisco's estimated 15,000 to 18,000 addicts since coming on the scene about seven years ago, Stalcup said. It sells for about $20 a bag that can be broken into several shots.

Stalcup attributed its popularity to the ease of bringing it into the country in ''body pack'' - bags attached to one's body - compared with the difficulties of smuggling so-called China White from Asia.

A 35-year-old musician who spoke with The San Francisco Examiner while recovering at San Francisco General said he uses heroin about once a week.

''It hit me good. Real good,'' he said. ''I remember telling my friends that it almost was too good. I sat back to enjoy it and just nodded off. Next thing I was waking up here.''

While San Francisco police pursued leads from overdose victims, Oakland police seized 4 pounds of black tar heroin worth about $2 million, and arrested five people in their largest heroin bust in more than 10 years.

Last week, federal Drug Enforcement Administration agents seized 2 kilos of black tar heroin at hotels near San Francisco International Airport.