Mallon is to serve a minimum of 25 years, Quinn 24 years and Casterton 22.
Mr Oldham’s body was found “covered with extensive injuries” at the property in Dolcoath Road on 1 April 2014.
They included the “worst kind” of internal infection, “deep bruising” to his sexual organs and thighs, four fractured ribs and broken fingers.
The trial heard Mr Oldham was “routinely bullied” by the three housemates who acted “like a pack of dogs”.
They urinated on him, force fed him dog faeces, prevented from going to the toilet and stole his money and possessions.
The court was told the trio considered him “fair game” after discovering he had indecently assaulted a girl under 14 in 1987.
Mallon and Casterton, both of Grenville Road, Falmouth; and Quinn, of Tregullow Road, Falmouth, were also found guilty of allowing the death of a vulnerable adult.
Judge Graham Cottle had previously directed the jury to find Richard Rosevear, a schizophrenic heroin user, not guilty of murder owing to “insufficient evidence”.
The jury found Mr Rosevear, 33, of Truro Road, St Austell, not guilty of allowing the death of a vulnerable adult.
Judge Cottle told the jury that, after Mr Oldham’s death, Mallon, Casterton and Quinn laughed about bullying him “as if they didn’t have a care in the world” and that Mallon was recorded saying “ding dong the nonce is dead”.
The court heard that Mallon had appeared “cut up” to police about what had happened, and had said in a statement that Mr Oldham “had been like a father to him”.
However, describing Mr Oldham as being “brutally and horrifyingly attacked”, he told Mallon: “I am satisfied that your role was the main role. He was viciously assaulted”.
Judge Cottle thanked the jury for hearing a “tough case” with “some sordid details”.