Three US teenage boys have been charged with beating to death a 57-year-old homeless man in Philadelphia.

Victim Kevin Cullen was found on the ground unconscious in the city's Holmesburg neighbourhood by police responding to reports of a robbery on 26 November.

He was taken to a nearby hospital but died from his injuries the following day.

Officials say Cullen's pockets were turned out and empty, leading them to suspect the motive was robbery.

On Sunday (3 December), Brandon Conrad, 14, and Malik Page, 16, were charged with murder, robbery and related offences in connection with Cullen's death.

Emmanuel Harris, 17, is charged with attempted murder, aggravated assault, robbery and related offenses.

The teens, all from Holmesburg, were charged as adults. Detectives are also trying to determine if the three boys were also involved in another assault.

Cullen grew up in Philadelphia's Mayfair neighbourhood and graduated from the city's Father Judge High School in 1979, Philly.com reported.

He was said to be a regular "fixture" of the neighbourhood.

"He'd be somebody you'd see on the street all the time, and maybe you'd help him out, maybe you wouldn't," his younger brother Mark Cullen, 54, told Philly.com. "But he wasn't aggressive or mean or any of those things."

In shock over the brutality of his brother's death, he added: "It's going to haunt me for a long time."

Tom Cullen, the eldest of Kevin's five siblings, said his now dead brother was "intelligent and funny, even through his ups and downs."

Kevin had worked a variety of jobs during a prolonged battle with alcohol abuse, he added. He loved rock music and bands like Kiss and the Who.

A vigil is planned for 6 pm Wednesday on the 4200 block of Loring Street, where Kevin was attacked.