"It has just started. All three players are here," an International Cricket Council official told AFP from the closed door hearing at the Qatar Financial Centre.
It is scheduled to run until January 11, although lawyers have indicated a verdict may come earlier.
The three face charges of spot-fixing during Pakistan's tour of England last year in a scandal that rocked the sport. It is alleged that they conspired in the bowling of deliberate no-balls -- claims they all deny.
Amir entered the building first, followed soon after by Asif. They came separately by car. Both wore dark suits, but no ties and did not speak with the media.
Butt arrived just before the hearing was due to start, also in a suit and open collar.
The three-man independent hearing is being led by code of conduct commissioner and leading lawyer Michael Beloff of England, aided by Justice Albie Sachs from South Africa and Kenyan Sharad Rao.
Rao told waiting journalists that he felt the future of cricket was bright.
"It is very important for cricket but I can't comment very much on it because we haven't even started," he said.
"The future of cricket is good because I think that's what this exercise is about. So it should be a very clean game and we can rely on the result of this."
"Spot-fixing" controversy timeline
August 29, 2010
The News of the World says it paid 150,000 pounds (230,000 dollars) to a middle man in return for details about the timing of three no-balls in Pakistan's fourth Test against England at Lord's.
The report says Amir Asif delivered the blatant no-balls at exact points in the match agreed with the alleged fixer.
The trio are interviewed by Scotland Yard detectives.
News of the World publishes photograph of the alleged middle man, Mazhar Majeed, counting wads of banknotes given to him by a reporter posing as a front man for a betting syndicate. Majeed is later bailed.
August 30
There are suggestions that Butt, Amir and Asif could be withdrawn from the Pakistan team to ensure that two Twenty20 internationals in Cardiff go ahead.
August 31
Pakistan Cricket Board (PCB) says it will not suspend its players while investigations continue.
Butt, Amir and Asif summoned to meet the chairman of the Pakistan Cricket Board, Ijaz Butt, and the Pakistan high commissioner, Wajid Shamsul Hasan, in London.
September 2
Salman Butt, Amir and Asif dropped from the Twenty20 games.
The three vow to clear their names, according to Hasan, who adds that they are pulling out of the tour because of the "mental torture" of the scandal.
ICC charges Butt, Amir and Asif with various offences under its anti-corruption code. All three are provisionally suspended.
September 3
Ronnie Flanagan, chairman of the ICC's Anti-Corruption and Security Unit, tells reporters: "The conclusion that we have come to is that there is a really arguable case to answer."
September 4
Pakistan one-day skipper Shahid Afridi apologises for the "spot-fixing" row, saying: "On behalf of these boys -- I know they are not in this series -- I want to say sorry to all cricket lovers and all the cricketing nations."
News of the World claims a fourth Pakistan player is being probed over the claims, but declines to name him for "legal reasons".
September 5
News of the World releases footage of Pakistan Test player Yasir Hameed in which he claims team-mates "were doing it (fixing) in every match".
September 17
Police pass the "spot-fixing" file to the Crown Prosecution Service.
September 18
ICC launches investigation into the third one-dayer at The Oval -- won by Pakistan -- after receiving information from the Sun tabloid on allegedly pre-arranged scoring patterns. It later emerges that the ICC tried to persuade the ECB to call off the Oval match shortly before the start.
September 19
PCB chairman Ijaz Butt alleges England were paid "enormous amounts of money" to lose deliberately at The Oval.
September 20
England team threaten to sue Ijaz Butt.
September 22
England, having been pulled back from 2-0 up to 2-2, beat Pakistan by 121 runs at the Rose Bowl to take the five-match series 3-2 in the final fixture of the English season.
September 29
Ijaz Butt withdraws allegations that England players had "thrown" the third one-day international.
October 4
Chairman of non-league English football club Croydon Athletic David Le Cluse, 44, found dead from gunshot wounds in a garage near his home in Sutton, south of London. The owner of the club is Mazhar Majeed.
October 13
The ICC says there is no evidence of corruption in the third one-day international between Pakistan and England.
October 22
Asif withdraws challenge to his provisional suspension.
November 4
Pakistan suspends the contracts of Salman Butt, Mohammad Asif and Mohammad Amir.
December 13
Salman Butt denies allegations he was involved in a spot-fixing scam, saying: "I have not done anything such as this in all my life or cricketing career".
January 6
A three-man independent anti-corruption tribunal against Butt, Asif and Amir opens in Doha, led by code of conduct commissioner and leading lawyer Michael Beloff of England.
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