Mohammed Abikar, Hassan Edin Hassan and Tanzanian Rashid Charles Mberesero have been found guilty of carrying out the Garissa University College attack in 2015.
They will be sentenced on July 3.
The fourth suspect, Sahar Diriye, was acquitted.
The four suspects appeared before Milimani law courts chief magistrate Francis Andayi.
On April 2, 2015, al Shabaab militants descended attacked Garissa University College, killing 148 students.
The attack also left 83 people with serious injuries.
The National Intelligent Service and National Police were accused of not doing enough to prevent the attack.
The Office of Director of Public Prosecutions closed its file on the Garissa University attack case on April 2, 2019.
Four out of the five suspects were found to have a case to answer.
A total of 22 witnesses testified against them, including students who survived the attack.
