The prison officer was attacked by Hashem Abedi, 24, Ahmed Hassan, 22 , and Muhammed Saeed, 23 on 11 May 2020.
The victim suffered facial and head injuries, bruising, and received hospital treatment for damage to his hearing.
The three men accused were found guilty of actual bodily harm. Abedi was also found guilty of assault on an emergency worker, relating to a second prison officer who helped to stop the attack.
Abedi was sentenced to three years and 10 months’ imprisonment, to be served consecutively to his existing jail term.
Saeed and Hassan were sentenced to three years’ imprisonment, to be served consecutively to their existing jail terms.
Officers spoke to witnesses, interviewed the three men, and worked closely with staff at HMP Belmarsh to establish the circumstances and bring forward the prosecution.
Due to the criminal history of the defendants, the assault was investigated by the Met’s Counter Terrorism Command. However, there was no evidence of any terrorist intent.
From their enquiries, officers believed the three men carried out the attack because they were upset at having lost certain prison privileges, and were of the view that other inmates were treated more preferably.
Commander Richard Smith, who leads the Met’s Counter Terrorism Command, said: “These three offenders launched a cowardly and vicious attack on a prison officer, in what they saw as retribution for having some of their privileges taken away.
“One witness described it as a ‘frenzied’ attack, and I have no doubt they set out to seriously injure their victim. I would like to praise the prison staff who intervened very quickly, and were able to stop the attack and gain control of the situation.
“We worked closely with the prison service to thoroughly investigate this incident, and ensured these men were put before the courts.”
On 11 May 2020, Abedi complained about his privileges being downgraded during a meeting with the prison unit’s governor and the prison officer who made the decision to remove his privileges.
Later that day, the three men were being taken back to their cells after a period of exercise in the yard when, after waiting in a corridor for a few seconds, they all ran towards the prison officer’s office.
The victim was kicked in the head by Saeed. Abedi was seen picking up and swinging a chair at him. Kicks, punches and headbutts were thrown in the brief attack.
Abedi, Hassan and Saeed were charged in January 2020, and first appeared at Westminster Magistrates’ Court on Wednesday 7 and Thursday, 8 April 2021.
They were found guilty of the offences on Tuesday, 22 February, following a trial at Woolwich Crown Court.