Biswajit Majhi, Abhisekh Roy and Subhankar Das were arrested in connection with the case, police sources said.
The plot to kill the woman, Chaitali Santra, was hatched by a man named Debashis Dey, who was jailed after the human rights activist helped his wife, a neighbour, in a marital dispute, the sources said.
In jail, Dey came in contact with Majhi, another prisoner.
After coming out of jail, Dey allegedly paid Majhi Rs two lakh to kill the woman activist, they said.
Majhi with the help of Roy and Das, the other two accused, put together a parcel bomb which was hand delivered to the woman on September 26, the sources said.
The woman was killed and her husband was injured when she tried to open the parcel bomb.
Dey is at present in Maharashtra and has been detained by the police, sources said. A team of Howrah police was on its way to Maharashtra to bring him back, they said.
