The three were identified as Yousif Rezaii, 30, Ali Vahabi, 33, and Kamran Saharkhiz, 35.
It has been a while that the prosecutor and executing director of the prison in the city of Rasht has been forcing prisoners to watch the execution of their cellmates.
The court in this city also issued a death penalty for an architect student at Rasht University.
Arman Parvizi, a Kurdish activist, has been in prison from early 2013. He is convicted of action against the national security and waging war on God by cooperating with a Kurdish group.