Officials said they think the boys, ages 5 to 11, are too young to have wandered very far since last being seen Wednesday evening. The river is about three blocks from the home where they were playing.
Police have used helicopters and search dogs to scour the neighborhood, and volunteers handed out fliers to passing cars displaying photos of Jesstin Pagan, 5; Daniel Agosto, 6; and Anibal Cruz, 11.
There's no evidence of abduction, CBS News Correspondent Jim Axelrod reports. Dogs may have picked up the boys' scent on the nearby riverbank.
"We've done everything we can. The only thing we can do now is hope and pray we'll turn around and they'll be walking down the sidewalk and saying, 'We're here,'" said Jennifer Calo, an aunt of Cruz.
Police Capt. Harry Leon said Cruz has the mental capacity of a younger child, part of the reason officials think the trio stayed in the area. The boys were playing in the side yard of Cruz's home at about 5 p.m. when they were last seen by relatives. Agosto lives across the street.
"I turned my back for two seconds and they were gone," said Jessica Pagan, Jesstin's mother, who brought her son from their home in nearby Mount Ephraim to the city to visit friends.
Police said they thought about issuing an Amber Alert - the alert that goes out when children have been abducted - but there wasn't enough evidence to support it, Axelrod reports.
Local merchants told relatives the boys were seen at a water-ice stand, a pizza parlor and a store Wednesday evening
The area is about three blocks from the Delaware River. Leon said the fire department planned to search there.
Late Thursday morning, as residents turned out to help, a police search dog was followed through the streets by officers, reporters and onlookers, including children on bikes and mothers with strollers.
Police Chief Edwin Figueroa said authorities had received several reported sightings of the children in different parts of the city.
The home from which they disappeared is a few doors from a school and in a neighborhood that is safer than most in Camden, which last fall was ranked the most dangerous city in the United States in a reference book that compares crime statistics.