Two of the spies - Maksim Gruzdev and Aleksandr Rudnev - are Estonian citizens, while one Pavel Romanov's citizenship was not revealed.
According to ERR's Russian-language service, the three were smugglers with no access to state-level secrets.
Harrys Puusepp, press secretary of the Estonian Internal Security Service (KAPO), told ERR that the criminals "Managed to do some damage, though not enough to pose a danger to the constitutional order."