Joe Pooley, 22, who needed the help of adult social care, slept with Becki West-Davidson, 30, and she then “stoked up hostility towards him in the hours leading up to his death”, an earlier trial at Ipswich Crown Court heard.
Sebastian Smith, 35, from Hawick in the Scottish Borders and 31-year-old Sean Palmer, of Great Yarmouth in Norfolk, both attacked Mr Pooley and threw him in the River Gripping, Ipswich in August 2018.
Mr Pooley’s body was found in the river on August 13, 2018.
Detectives concluded that the 22-year-old had been unlawfully killed in the early hours of August 7 2018, and that his likely cause of death was immersion in water.
West-Davidson from Ipswich, and her two co-defendants denied murder but were found guilty following an earlier trial, Suffolk Police said.
West-Davidson was jailed for life with a minimum term of 17 years for Mr Pooley’s murder following a sentencing hearing on Monday.
Smith and Palmer also received life sentences.
Smith must serve 21 years before he is considered for release and Palmer must serve 18 years, police said.
Detective Inspector Karl Nightingale, of Suffolk Police’s Major Investigation Team, said: “Joe Pooley was a young man who wanted to be liked and sought friendship.
“Sadly, some people took advantage of his vulnerabilities and used him.
“The convictions and sentencing of Sebastian Smith, Sean Palmer and Becki West-Davidson bring to a close a long wait for some justice for Joe’s family.
“They have shown great resilience and patience throughout whilst also grieving the loss of a much-loved son, grandson, brother, uncle, family member and friend.
“My thoughts will always be with them.”
A fourth defendant, a 26-year-old woman, was cleared of murder and of manslaughter.