The three arrested persons were Biswajit Majhi, Abhisekh Roy and Subhankar Das, 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 Biswajit Majhi another prisoner.
After coming out of jail Dey paid Biswajit Majhi Rs 2 lakh to kill the woman activist, they said.
Majhi with the help of Avishekh Roy and Subhankar Das, the other two arrested, 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.
Debashis Dey was at present in Maharashtra and has been detained by the police, the sources said. A team of Howrah police was on its way to Maharashtra to bring Dey back back.