Paul Massey, 55, known as “Salford’s Mr Big”, was shot dead outside his Clifton home in July 2015.
More recently, John Kinsella, 53, a mob fixer, was shot dead while walking his dog in May this year.
Their unsolved murders have been investigated by police in Manchester and Liverpool.
The joint investigation has seen the arrest of two men and one woman.
Greater Manchester Police said a 37-year-old man was arrested at Manchester Airport Wednesday morning on suspicion of both the murder of Massey and the murder of Kinsella.
A woman, 39, was also arrested at the airport on suspicion of assisting an offender in relation to the Kinsella murder.
In Salford, a 48-year-old man was arrested on suspicion of conspiracy to murder in relation to the Massey murder investigation.
Massey spent 14 years in prison for stabbing a man in the groin in a nightclub and was a notorious figure well known to the police.
He went on to claim to be the victim of a witch-hunt after he was arrested and then released without charge on suspicion of money laundering.
Mr Massey stood unsuccessfully to be mayor of Salford in 2012.
Kinsella had been involved in a £41,000 robbery in Grantham, Lincolnshire in 2006, where thousands of pounds worth of detergents, crisps and Easter eggs were stolen.
During his trial, the jury heard how Mr Kinsella had stopped a gangster terrorising Steven Gerrard, who played for Liverpool at the time.
Detective Chief Inspector Carl Jones, of GMP’s Major Incident Team, said: “These are the first arrests that have been made in connection with Paul’s death and the investigation remains focused on establishing the truth and bringing answers to his family.
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“Although the murders of Paul Massey and John Kinsella remain separate investigations by individual police forces, we will continue to liaise with colleagues from Merseyside Police as further enquiries are carried out.”
Detective Chief Inspector Mark Baker, of Merseyside Police’s Investigations team, said: “Detectives from Merseyside have been carrying out extensive inquiries since the murder of John Kinsella just three weeks ago and have established a number of positive leads.”