Officers said two more bodies were found on Wednesday, after initially saying on Tuesday that one person had died.
The victims of the fire – which happened in a detached house on a residential street in Kirton, near Boston – were a 27-year-old woman and two men aged 24 and 32.
All three were British, police said, though they have not been named.
Lincolnshire Police said it was treating the fire as a “domestic-related murder investigation”.
Officers said they knew the identities of the deceased, and were not seeking anyone else in connection with the deadly blaze.
At the home on Wednesday afternoon, officers and fire investigators could be seen sifting through the wreckage.
The only part of the roof remaining were two brick gable ends, while every window on the upper floor had been shattered. Part of the first floor wall had collapsed completely.
Two other people who were taken to hospital following the fire have since been discharged.
David Danby, a parish councillor who lives nearby, told the Press Association: “We'd seen in the New Year, and then gone to bed.
"Then at about 3am, the dog kicked off a bit and my partner thought he just wanted to go out to the toilet.
"When we got downstairs, you could see the reflections of the light, from the fire.”
He continued: "It was the ferocity of it.
"We just could not believe how quickly it had got from one side of the upstairs to the other.
"It was like a tinderbox.”
Lincolnshire Police said in an updated statement on Wednesday: “The two people taken to hospital have since been released. In total there were five people involved in this incident, the investigation is ongoing and officers are not seeking anyone else as part of their inquiries.”
