The court said in its ruling on Thursday that the three, identified as Suprapto 35, Muhammad Solihin, 24, and Heru Setyanto, 29, were guilty of violating anti-terrorism laws.
The laws were enacted just weeks after the October 2002 bombings of two nightclubs in Bali that killed 202 people, mostly foreign tourists.
"The three defendants have been legally and convincingly guilty for helping the others to commit an act of terrorism," presiding judge Effendi said in his ruling.
"The defendants' act had triggered massive fears among the community," he said.
The three were found to have been involved in "transporting explosive materials" from Dumai district town in eastern Sumatra to Pekanbaru and Bengkulu, allegedly at the request of Toni Togar, another suspect in the Marriott bombing.
The explosive materials were then brought to Jakarta, where they were later assembled into a bomb and detonated outside the Marriott hotel on 5 August 2003, leaving 13 people dead and more than 100 others injured.
The three are among 20 suspects to be convicted in connection with the Marriott bombing. Four other suspects have been sentenced to terms ranging from three to 10 years in prison.
The three were found
to have been involved
Jakarta district court
More than 100 Indonesians have been arrested on suspicion since the Bali blasts and the Marriott bombing.
Three of them have been sentenced to death for their major roles in the Bali explosion, while nearly 30 others were convicted from three years to life in prison.