- Mr Andayi, however, freed Sahal Diry Hussein, saying he cannot be linked to the attack just because he was found in the company of one of the attackers.
Nairobi.Tanzanian national Rashid Charles Mberesero plus two other suspects were on Wednesday convicted over the 2015 Garissa University terror attack.
Nairobi Chief Magistrate Francis Andayi also found Mohamed Ali Abdikar, and Hassan Aden Hassan, were involved in attack that left 148 people dead, most of them students.
Mr Andayi, however, freed Sahal Diry Hussein, saying he cannot be linked to the attack just because he was found in the company of one of the attackers.
Prosecutors had proven "beyond reasonable doubt" that "they knew the plot," he said, but did not give further details of the alleged conspiracy.
The three convictions are the first to result from a long-running investigation and prosecution.
All four gunmen were killed by security forces. The operation's suspected ringleader, Mohamed Mohamud, also named "Kuno," a former professor at a Koranic school in Garissa, was killed in southwestern Somalia in 2016.
The Shabaab said he had been killed by "US crusaders".
The group is fighting to overthrow the internationally backed government in Mogadishu.
But it also regularly carries out attacks in neighbouring Kenya, which has troops in Somalia as part of an African Union force.
Additional reporting by AFP