Two children and an adult died after a van rolled into a pond while the driver was away asking directions to a barbecue.
Another child was in critical condition.
The driver was outside the van when it started to roll down a hill to a pond about 70 yards (64 meters) away, Fire Battalion Chief Ismael Pomales said. People nearby tried unsuccessfully to stop the van, and fire personnel could not reach the people inside it.
A scuba team removed the victims about 20 to 25 minutes after they went into the pond.
The children appeared to be under 5, Pomales said. They and the adult have not yet been identified.
"It's horrible," Pomales said. "We've had children die before. That's probably one of the toughest things for any rescue person to deal with."
The driver, who was in apparent shock, was being questioned by police.