Officers were called to the red-brick house in East Park Street, Leeds, shortly before midday today after a relative of one of the occupants raised the alarm.
Inside, police and fire crews found the bodies of two women aged in their 20s or 30s, who were thought to be in a relationship. They were found in the living room.
The body of a male friend or lodger, aged around 18, was found in the kitchen. The couple had three Rottweilers, which also perished.
It is not believed that there was any sign of violence and officers are understood to be working on the theory that the cause of death was smoke inhalation.
The fire had burnt itself out before the emergency services arrived and it was thought the people may have died in the early hours of Saturday.
A white police tent was put in place in front of the property, which is in a block of four homes set back from the terraced street. Uniformed officers were keeping a cordon in place around the home.
Forensic post-mortem examinations will be carried out to determine the cause of death.
The people who died have yet to be formally identified.
One neighbour, who did not want to be named, said of the women in the house: “They were sound. They had three Rottweilers which used to make me jump.”
Neighbour Sylvia Atikinson, 56, knew one of the women to say hello to.
“She was a nice young girl, very polite,” she said.
“All I have heard is that there was a small fire in the house. To hear that three people have died... you can’t imagine it.”
Another neighbour said: “I don’t know what has happened here. All I know is there has been a tragedy.”
Neighbours said emergency services began arriving in the street shortly after midday. Amy Smith, who lives yards from the property with her partner Jimmy Andrews, said: “It was a shock to see some many fire engines, forensic officers and all the other police activity. I just hope there is no foul play involved.
“This area has a bit of reputation with some people but I have always found it to be quiet.”
Earlier, Acting Detective Inspector Mark Atkinson, of Leeds District CID, said: “Our inquiries are at a very early stage and we are currently trying to establish the full circumstances surrounding the deaths.
“Initial indications are there has been a small fire at the premises.
“Forensic post-mortems will be held to establish how the three people found inside the property have died.
“We believe the deaths are likely to have occurred in the early hours of Saturday and we would like to hear from anyone who has seen the occupants of the address on Friday.”
Anyone with information is asked to contact Leeds District CID at Elland Road via 101 quoting log number 538 or March 27.