Belgian police said at the time the cell was planning to kill and kidnap police officers under orders.
"The group was on the verge of carrying out terrorist attacks to kill police officers in public roads and in police stations," federal magistrate Eric Van der Sijpt told a news conference following the raid.
Police found Kalashnikov assault rifles, explosives, ammunition and communications equipment – along with police uniforms that could have been used for the plot, he said.
All three men were from Molenbeek, the gritty immigrant area of Brussels and home to several of the jihadists that would go on to carry out the attacks in Paris in November and last month's suicide bombings in Brussels, which killed 162 people in total and was claimed by the IS group.
Police believe the suspected ringleader of the November 13 Paris attacks, Abdelhamid Abaaoud, was central to the Verviers events, giving orders to the would-be assailants by phone from Greece.
The main suspect at the trial will be Marouane El Bali, who is accused of attempted murder for firing at police during the gun fight, an allegation his defence team firmly denies.
"He was a small player and was absolutely not aware of any planned attacks," his lawyer Sébastien Courtoy told the Belga news agency in April.
Killed in the raid were Sofiane Amghar and Khalid Ben Larbi, who left Molenbeek to join the IS group in Syria in April 2014. The two then slipped back into Belgium to the Verviers hideout which is about 120 kilometres east of Brussels.
On Monday, El Bali will appear in court in Brussels along with six other defendants.
El Bali is in custody, along with co-defendants Souhaib El Abdi, Mohamed Arshad and Mahmood Hajni. Three other suspects were released on parole.
The four principal defendants are standing trial on charges of forming a terror organization. The public prosecutor considers them the leaders of the so-called Verviers cell, along with Omar Damache, who was arrested in Athens around the same time.
The majority of the cell's suspected members are not in court, however, and will be tried in absentia.
These include two Belgians, five Frenchmen, a Moroccan and a Dutchman, who are thought to be either fighting for the IS group in Syria, already dead or in hiding.
The trial is scheduled to last three weeks.