Khobaib Hussain, 25, and Naweed Ali, 29, both from Birmingham, and Mohibur Rahman, 33, of Stoke-on-Trent, were convicted at the Old Bailey yesterday.
They were arrested in August 2016 as part of an undercover police operation.
A fourth man, Tahir Aziz, 38, from Stoke-on-Trent, was also found guilty of the same offence.
They will be sentenced later today.
Police said that the men, who had all denied plotting terror attacks, were inspired by ISIS – the self-professed Islamic State in Iraq and Syria.
They were described as “dangerous men” who were committed to carrying out a “mass casualty attack”.
Officers stated that the case was “one of the most complex counter terrorism investigations in a number of years”.
The Old Bailey heard how gang members named themselves the “Three Musketeers” when sending each other encrypted messages on Telegram.
Evidence found on the men’s mobile phones and other electronic devices revealed how they all shared “radical and extremist beliefs” in messages.
They were arrested after counter-terrorism officers set up a front courier firm, called “Hero Couriers”, where Ali and Hussain were offered a driving job.
Ali arrived for his first shift at the fake firm last August, leaving his Seat Leon car at the Birmingham depot, where MI5 officers searched the vehicle with a view to bugging it.
However, inside the car they found a JD Sports bag containing an incomplete pipe bomb and meat cleaver with the word “kafir” (disbeliever in Arabic) carved on it.
They also found an imitation firearm, a 9mm bullet, cartridges, industrial tape and latex gloves.
The court heard how when police searched Aziz’s car they found a samurai sword next to the driver’s seat.
All four men were arrested on 26 August.
Fake firm set up
The men claimed the evidence was planted by an undercover officer, known as “Vincent”, who posed as the boss of the fake firm.
The undercover officer, who anonymously gave evidence, was cross-examined for 12 days and consistently rejected the allegations.
The jury heard how the defendants had been looking at “violent material” online, joined “extremist” social media groups, and bought new phones to assist them further their plans.
Both Hussain and Ali have previously been jailed for terrorism-related offences.
In 2011, they had tried to join an Al-Qaeda training camp in Pakistan.
However, when they returned to the UK they were arrested and the following year both pleaded guilty to engaging in conduct in preparation of terrorist acts.
The trial heard how both men first met Rahman – who had been convicted of possessing an Al-Qaeda magazine – while in prison.
Responding to the verdicts, Chief Superintendent Matt Ward, head of the West Midlands Counter Terrorism Unit, said: “Back in 2010 and 2011, they were inspired by Al Qaeda.
“Since they’ve come out of prison, they’ve been inspired by the Daesh ideology.
“They shared lots of material regarding that ideology, lots of very violent material, they were trying to encourage others to join them on that journey.”
He added that the men had “carefully planned and took steps to avoid drawing attention to themselves.”
Mr Ward also defended all his officers, including man who was accused planting evidence in court.
He said: “Throughout the trial the defence made a number of accusations.
“Many of those accusations were groundless, they went against the integrity of the undercover operatives, they went against the integrity of the wider investigation.
“What I would say is just because a defence lawyer says something in court, doesn’t make it true.”
Partially secret trial
The four-month trial was held partially in secret in the interests of national security and two anonymous witnesses gave evidence behind closed doors.
Sue Hemming, from the CPS, said the men “shared the same radical belief in violent jihad”.
She added: “Recent attacks have demonstrated the kind of horror these defendants could have caused had they not been stopped.”