Flamengo is the most popular club in football-crazed Brazil.
There was no word on the cause of the fire. One of the three injured, a 15-year-old boy, suffered severe burns and was transferred to a specialist hospital, reports said.
A blaze has engulfed a dormitory at the youth team training centre of one of Brazil's biggest football clubs, killing 10 people, local media report.
A formal investigation into the fire is expected to begin shortly, according to the government of Rio de Janeiro.
Overhead images captured by an AP drone showed a charred area with smoke emerging.
One young player told reporters the fire started in his room.
The blaze broke out at 5:00 am in the Vargem Grande district of Rio in a modern facility where the top-flight professional Flamengo squad also trains, the news website G1 said. "God bless everyone's family". "It is a sad day for Brazilian football".
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Zico, who is outside Brazil, wrote on Instagram: "What a shock getting this news here on the other side of the world", the former Brazil midfielder wrote.
As news of the fire broke, several teams and players expressed their condolences on Twitter. "Lives are gone. May the Black and red nation have the strength and faith to overcome this moment".
Messages to Flamengo officials were not immediately answered.
However, numerous lodgings have a reputation for poor health and hygiene standards. In recent years, major clubs such as Vasco da Gama, Botafogo and Parana have hit the headlines due to the precarious conditions of their training centres.
After years of financial difficulties, Flamengo last year spent £4.7million to expand the Ninho do Urubu facilities.
At one point a shot was heard ringing out from a house and moments later two heavily armed officers were seen on a pickup truck carrying two bodies shrouded in blood-soaked sheets.