Eight others were jailed for five years.
The court of Gaibandha District and Sessions Judge Dilip Kumar Bhowmik fined the convicts Tk 1 lakh each.
Those who have been sentenced to death are – Shahriar Sarker Ridoy, Rakibul Hasan Sajib and Mahmudul Hasan Zakir.
Other convicts are – Masud Prodhan Sujon, Al Amin Islam, Rabeya Begum, Al Amin, Shimul Mia, Runa Begum, Jahangir Alam and Joynal Abedin.
According to case details, the convicts called Ashiqur Rahman Shammo out of his house on September 24, 2015 and killed him. Shammo was the only son of Gobindaganj Mayor Ataur Rahman.
Political vengeance was the motive, Rahman said. He filed a case against 11 people the next day with Gobindganj police.
After prolonged investigation, Officer-in-Charge Mojibor Rahman pressed charges against the accused in court.