Lalmonirhat District and Sessions Judge Mia Ali Akbar Azizi also slapped a Tk 5,000 fine on each of the convicts on Thursday.
Moniruzzaman Iqbal alias Selim and Mostafizar alias Mosta, the two given life terms, were present in court.
The third convict, Sadiqul Islam Sadiq, is on the run.
The convicts will have to spend an additional six months in prison if they fail to pay the fine.
The militant organisation Jamaa’tul Mujahideen Bangladesh (JMB) orchestrated a series of blasts on Aug 17, 2005 across Bangladesh. They exploded bombs at four locations in Lalmonirhat.
Police had accused unidentified men for the incident.
A year later, police pressed charges in court against Selim and Mosta.
A supplementary chargesheet that included Sadiq was filed in 2007 after the prosecution moved court.