The three were part of a patrol fencing off a crevice following a blizzard at the Mammoth Mountain ski area, The Los Angeles Times said. Seven other people were injured in the incident.
One of the dead, a researcher from the University of California-Santa Barbara, was an expert in snow hydrology and remote sensing of snow. He was killed when he jumped into the vent in an effort to help team members, the report said.
The vents, which emit a rotten-egg smell, are relatively harmless in summer when the gas dissipates into the atmosphere.
"The problem comes in the winter when the snow builds up and covers this thing," David Hill, the scientist in charge of the U.S. Geological Survey's observatory in the Mammoth area, told the Times. He said the heated gas can melt snow, forming a cavity below the snow surface, endangering skiers.