The three men jumped into the Sacramento-San Joaquin River Delta on Sunday near Three Mile Slough Bridge, the Rio Vista Fire Department (RVFD) said.
According to the Los Angeles Times, the child was a relative of the three men who all jumped into the water at the popular swimming and boating spot at the Brannan Island State Recreation Area.
Relatives told officials that while the child was pulled out of the water and survived, none of the three men were seen resurfacing.
The fire department said in an update on Sunday night that “the incident has transitioned to a recovery and three adults remain missing. RVFD was released by law enforcement. We have no further information.”
Speaking to reporters, fire battalion chief Brandon Wilson said the current at Three Mile Slough Bridge can sometimes be “pretty bad”, and that the nearby recreational park had been closed to swimmers since April.
“It seems to be calmer but the undertow and the currents in that area are pretty bad,” he said to the Times. “They saw them go under and never saw them come back.”
Mr Wilson said while the Brannan Island State Recreation Area had been reopened to boaters after a management company ended its contract, there was no life jackets available for swimmers.
“There was more of a swimming area towards the back that was a lot calmer,” he said. “Now people are basically finding areas and just going in the water, probably not realising how deep it is.”
An investigation is ongoing.