Those convicted are Jamal Khan, his brother Khalilur Rahman and Sohrab Hossain.
Additional District and Sessions Judge Mohammad Bazlur Rahman announced the verdict on Wednesday with the accused present in the dock.
They were found guilty of killing a farmer, Monsur Ali Khan, by slitting his throat in Nolchhiti municipality of the district town in 2000.
Two other accused in the case, Rustom Khan and Zakir Hossain, had been acquitted, said state’s counsel M Alam Khan Kamal.
They killed the farmer by slitting his throat, he said quoting from the case dossier.
“Monsur’s son Abdul Majid Khan and daughter Nilufa Begum also suffered stab wounds.”
Abdul Majid started the case the next day accusing seven people. The court transferred the case to the Criminal Investigation Division or CID.
The charges were framed against five in 2015.