Prosecutors in the trial of three men accused of preparing to carry out a terrorist act in Melbourne's CBD have told court the accused were motivated by extremist jihadist ideology.

Abdullah Chaarani, 27, Ahmed Mohamed, 25, and Hamza Abbas, 23, were charged with conspiring to carry out preparations for a terrorist attack in the CBD during the 2016 Christmas period.

The trio were committed to stand trial last year.

In his opening address on the first day of the trial in Victoria's Supreme Court, Nick Papas QC said the trio followed an extreme brand of Sunni Islam that directed followers to wage war against non-believers.

"They were motivated by ideological, religious objectives to raise violent jihad against those who are non-believers," Mr Papas told the jury of eight women and seven men.

"They believed they would be sent to paradise as a martyr."

Mr Chaarani shook his head during the remarks made by Mr Papas.

The jury was also shown CCTV footage of the three men gathering in Federation Square on December 20, 2016, talking in a small group.

The crown alleges it was a reconnaissance mission, and is evidence to suggest the trio took overt acts or further steps to carry out an attack.

The jury was shown a video of the three men in Federation Square. ( File shot: Photograph by Matt Smith )

The court heard Mr Chaarani is the cousin of Mr Abbas, and Mr Mohamed is a friend of Mr Chaarani.

The jury was told they could expect volumes of evidence, and that the trial could go for up to 10 weeks.

"You will hear from experts, you will hear they purchased metal pipes, light globes and batteries to make IEDs, that they took steps towards manufacturing these, conducted recon on Flinders St and attempted to have firearms" Mr Papas said.

"We have intercepted telephone messages between accused and others who are uncharged. There's been emails and texts sent."

Lawyers defending the men have not yet responded.