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