The accident was reported shortly before 8 a.m. at a six-story building under construction at 19th and Illinois streets.
San Francisco fire Battalion Chief Charles Crane said the accident happened as workers were pouring concrete on the building's roof.
The roof support structure gave way and collapsed onto three workers below, who were in a stairwell on the sixth floor, he said. The impact caused them to fall three stories.
The fire department responded and found the three workers covered in concrete that hadn't yet hardened, Crane said.
Crane said the weight of the concrete was "equivalent to having a large wall fall on you."
The three were severely injured but were conscious when they were taken to the hospital, he said.
A fourth worker was also hospitalized after suffering minor injuries while trying to help the workers who fell, Crane said.
San Francisco General Hospital spokeswoman Rachael Kagan said the workers are all men in their early 20s to late 30s. Three are in serious condition and the fourth is in fair condition.
The construction is happening on the site of the Potrero Launch project, a 196-unit, mixed-use development slated to be completed next year.
The California Division of Occupational Safety and Health is investigating the incident.