All three are residents of Bhandup.
Crime Branch officers said the activist, who is a member of an NGO run by a retired police officer, was attacked with iron rods in Janata Market, Bhandup, on October 25 last year.
Bhandup Police had initially arrested Dilip Vaishnav and Vilas Kadam for the crime, but they were granted bail for lack of evidence.
The activist subsequently wrote to senior police officers seeking a stronger probe and the case was transferred to Crime Branch unit 7 earlier this month.
“We questioned nearly 50 people, including neighbours of the victim and those with whom he had feuds, and zeroed in on three.
“The suspects confessed to the crime and were arrested,” said an officer.
“Lad and Kadam are brothers-in-law. Kadam and the activist stay in the same building.
“The activist had filed a court case against Kadam for running an unlicensed security agency in the residential building.
“Kadam lost the case and had to shut down the enterprise, after which he planned the attack with Lad,” the officer said.