Anton Levin, 24, from Dagenham, east London, was found stabbed to death at a flat in Southend, Essex, in November.
Danielle Russell, 26, and two teenagers were found not guilty of murder but guilty of manslaughter, at Chelmsford Crown Court.
Russell and the two boys - one aged 16 from Thurrock, the other 17 and from Basildon, will be sentenced on 3 July.
All three were also found guilty of conspiracy to commit grievous bodily harm on another man and guilty of conspiracy to commit robbery.
They were found not guilty of the attempted murder of another man, aged in his 20s.
Mr Levin was stabbed and died at Russell's Southend flat and another man was injured.
Det Ch Insp Simon Werrett said: "The jury have heard the evidence and have decided Danielle Russell and the two youths were responsible for the death of Anton Levin.
"This was another unnecessary death linked to gang activity in Essex. The defendants have never any shown remorse for the attack and the needless loss of life."
A family statement said: "He was a young educated man who had just graduated from Greenwich University and had a bright future ahead of him in the property market.
"I feel for such a bright and intelligent young man he had his whole life ahead of him, which was cut too short. No one deserves to go so young.
"He was very protective of me and helped me to bring up his little sister too. "
A woman and two teenage boys have been found guilty of killing a man who had recently graduated.