Comilla's Fourth District and Sessions Judge Noor Nahar Begum Shiuli passed the verdict on Wednesday.
The convicts are Shipon Majumder alias 'Ripon', 22, 'Shahabuddin', 22, and 'Taslima' aka 'Salma', 26.
Shahabuddin and Salma are behind the bars while Ripon is on the run, Public Prosecutor Mostafizur Rahman Liton said.
According to the case dossier, Shamsul Huda alias Shamsu, 38, of Chouddagram in Comilla, was abducted on Dec 9, 2013. The abductors demanded Tk 250,000, but as the family failed to pay the ransom, the abductors killed Shamsu after three days.
Salma was arrested a few days later in the case filed by Shamsu's family. Shahabuddin and Ripon were arrested later following the information given by Salma.
The charge sheet naming the trio was submitted to the court in May 2014. The court took statements of 18 of the 22 witnesses.