The fatal shooting occurred on Dec. 22 near Eighth Lane and Linden Avenue at about 7:15 p.m., police said.
Two victims, Omar Cortez, 18, and Gonzalo Avalos, 19, died at the scene. Hector Flores, 20, was taken to San Francisco General, where he died on Thursday.
Three other men were wounded, but they are expected to survive.
The three survivors have not been cooperative with investigators, and few witnesses have come forward, South San Francisco Police spokeswoman Sgt. Joni Lee said Tuesday.
The reward and tip line are the department's latest efforts to gather information about the shooting. The money is being offered to information that leads to an arrest and conviction in the case.
South San Francisco Mayor Kevin Mullin said the police stepped up patrols in the neighborhood, and investigators are aggressively pursuing all leads.
"We need neighborhood members to come forward with any information that can be helpful," Mullin said.
Anyone with information about the shooting is asked to call the tip line at (650) 829-3700.