"It's very, very tragic, and the investigators are working hard to get leads and identify witnesses and suspects," police spokesman Sgt. Troy Dangerfield said this morning.
The shootings occurred early Thursday in the Tenderloin neighborhood, Thursday evening in the Oceanview neighborhood, and early this morning in the Bernal Heights neighborhood.
Dangerfield said there was no immediate information indicating the shootings were related.
In the latest shooting, police responded to a report of shots fired near the Alemany housing projects near Alemany Boulevard and Ellsworth Street, just north of Interstate Highway 280, at about 12:10 a.m. today.
The victim, identified this morning by the medical examiner's office as 20-year-old Raymond Twine, was found shot in front of the housing projects and pronounced dead at the scene, police said.
The medical examiner's office is still trying to determine where Twine lived.
On Thursday evening, a 19-year-old man was shot at about 6:40 p.m. at Capitol Avenue and Lobos Street, according to police. The man, whose name has not yet been released, died at San Francisco General Hospital.
Officers detained several suspects and recovered two guns at the scene, police said.
Investigators are probing whether Thursday evening's shooting is gang-related.
Dangerfield said the Oceanview neighborhood is known for gang activity, but added it's unclear whether gangs were involved in this incident.
Earlier Thursday, a 26-year-old man was gunned down in the 400 block of Jones Street, near Ellis Street, at about 12:20 a.m.
The victim, identified today as Steven Garrett Jr., of San Francisco, reportedly had argued with three men and was shot multiple times after he punched one of them in the face. He was pronounced dead at the scene.
The three suspects - described only as black men about 20 years old, one reportedly with dreadlocks - ran north on Jones Street and then west on O'Farrell Street, police said.
Investigators found a semiautomatic handgun believed to have been used in the shooting stashed in a mailbox in the 500 block of O'Farrell Street, according to police.
No arrests have been made in any of the shootings, San Francisco's 31st, 32nd and 33rd homicides this year. At the same time last year, there were 31 homicides.
