Mikael Davud, a member of the Chinese Uighur minority and Norwegian national believed to have ties to Al-Qaeda; Shawan Sadek Saeed Bujak, an Iraqi Kurd residing in Norway; and David Jakobsen, an Uzbek also living in Norway, denied the charges of "conspiracy to commit a terrorist attack" and "possession of materials used to make explosives".
According to the prosecution, the trio first planned and prepared an attack against the newspaper Jyllands-Posten, and then the target switched to caricaturist Kurt Westergaard whom they planned to kill.
Westergaard, 76, drew the most controversial of the 12 cartoons, featuring the prophet Muhammad with a lit fuse in his turban, which touched off a wave of violent and sometimes deadly protests around the Muslim world.
The three were arrested in July 2010 after procuring chemicals used to make explosives. Police found hydrogen peroxide and acetone stored in a cellar belonging to Bujak.
According to Norway's intelligence agency PST, 40-year-old Davud, presented as the mastermind, had ties to Al-Qaeda which trained him in explosives handling at a camp in Pakistan between November 2008 and July 2010.
"This adds to the gravity of the case, by giving it an international dimension," prosecutor Geir Evanger told AFP.
"The fact that an international terrorist organisation has turned its glance to northern Europe is worrying and shows that this is not just kids' games," he said.
According to Evanger, weapons manuals, instructions on how to make explosives, calls for a "jihad" (holy war) and a video of Ayman al-Zawahiri, now Al-Qaeda's chief, were all found on a multimedia device.
In his address book, police also found e-mail addresses for people it believes to be linked to Al-Qaeda and who were suspected to have been in contact with the plotters of an attack on the New York subway.
Davud has meanwhile denied any ties to Al-Qaeda. Through his lawyer, he said he had never travelled to Pakistan since arriving in Norway in 1999.
In police interrogations, Davud and Bujak, 38, both held in custody since their arrest, have admitted they were planning an attack, though their versions have differed on the target.
They have both rejected the claim that their plans constituted a terrorist attack.
Davud has said he was planning an attack on his own on Chinese interests, and that his motive was personal vengeance.
Bujak meanwhile said the target was the Jyllands-Posten newspaper and Westergaard, but through his lawyer explained that he had only vague plans that could not be considered a terrorism plot.
The third man, Jakobsen, who contacted police voluntarily in November 2009 and is the only one of the three to speak Norwegian, has denied any responsibility and is currently a free man.
The three men risk up to 20 years in prison if found guilty.
The trial opened less than two weeks after the offices of French satirical weekly Charlie Hebdo were firebombed in Paris as it published an edition featuring the prophet Muhammad as "guest editor" on the cover.
The trial is scheduled to continue until late December and a verdict is expected early next year.