Butt criticised the ICC for victimising Pakistan cricketers when his appeal was rejected, while Asif - who did not appeal - has made no statement since the suspension.
Thursday's proceedings began with a statement from the ICC presenting its case against each of the players, before Amir, Butt and Asif will be allowed to respond.
When those formalities are complete, the ICC will then detail its evidence, which will include testimony from witnesses both at the hearing and by telephone. Both the players and tribunal members can question witnesses.
The players will then present their defence, with witnesses again available for cross-examination.
Pakistan's Twenty20 and one-day captain Shahid Afridi will join coach Waqar Younis as witnesses at the request of the ICC, however it is not clear whether either's testimony will be used against their former colleagues.
Following closing statements, the tribunal will retire to deliberate on its verdicts before any sanctions are decided upon. How and when these might be announced to the players or the public has yet to be decided upon, although the ICC has said the players will learn of any punishment verbally "if time allows".
Speaking to the BBC in December, Lorgat said that the trio will be entitled to appeal against any ruling at the Court of Arbitration for Sport.
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