Protestors from across Europe are expected in Luxembourg as the three men begin the appeal to a verdict that saw two of the defendants receive jail sentences last June.
The LuxLeaks scandal sparked a major global push against generous deals handed to multinationals that grew even stronger after the Panama Papers revelations earlier this year.
Former PricewaterhouseCoopers employees Antoine Deltour and Raphael Halet in June received 12-month and nine-month sentences, respectively.
The re-trial, following an appeal by Deltour and Halet, is due to last from Monday until December 21.
In his June ruling, Judge Marc Thill recognised the defendants' "undeniable contribution... To greater transparency" on tax matters.
But the two men were still found guilty, angering activists who have launched petitions and raised money for legal fees.
"The original sentences of Mr Deltour and Mr Halet were a total disgrace, and we want this injustice to stop," said Tove Ryding, a tax justice coordinator at the European Network on Debt and Development.
The blockbuster leak revealed the huge tax breaks that tiny EU nation Luxembourg offered international firms including Apple, IKEA and Pepsi, at a time when Jean-Claude Juncker, now head of the European Commission, was prime minister.
The revelations ended up prompting the EU to take urgent steps to stop global firms avoiding tax in Europe, including anti-trust inquiries into firms like Apple, McDonald's and Amazon.
The scandal also pressured Luxembourg into accepting a new law that requires EU member states to share tax deal information with its bloc partners.
The documents were originally used for a 2012 report by reporter Perrin on French public television but really exploded onto the world stage two years later with the huge "LuxLeaks" release of all 30,000 pages into the public domain.
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