Former PwC employees Antoine Deltour, 31, Raphaël Halet, 40, and journalist Edouard Perrin, 45, face charges that could mean five to 10 years in jail and fines of over one million euros.
Deltour, who was a PwC auditor before leaving in 2010, is accused of stealing documents from the accounting firm's database, revealing business secrets, violation of professional secrecy and money-laundering.
Halet, who is accused of being behind a second leak, faces the same charges, while Perrin is charged with being an accomplice in all the offences.
The documents downloaded by Deltour when he quit the company became the subject of a story by Perrin for the Cash Investigation programme on state-owned France 2 TV in 2012 but the story hit international headlines when the International Consortium of Investigative Journalists (ICIJ) shared 28,000 documents with international media in the Luxleaks dump in 2014.
Coming at a time when Luxembourg's then prime minister and finance minister Jean-Claude Juncker was president of the European Commission, they showed that 340 companies had benefited from "tax rulings", commitments by the country's authorities that they would pay as little as one percent tax if they registered in the Grand Duchy.
At one time 2,000 such rulings per year were signed by the country's tax officials, according to the magazine Paperjam.
Luxembourg is not the only country to practise tax rulings.
Support for whistleblowers
The trial opens in the wake of new tax-dodging revelations in the Panama Papers documents dump, and NGOs and politicians have dubbed the defendants courageous whistleblowers.
Anti-corruption campaign Transparency International has written to PwC, calling on it to drop the charges, while the ICIJ has called Perrin's prosecution a blow to press freedom.
Aid group Oxfam, which wants a crackdown on tax-dodging, says that the defendants should be "celebrated not prosecuted" and some 100,000 people, including fellow whistleblower Edward Snowden and Wikileaks boss Julian Assange, have signed a petition supporting them.
In an open letter, French journalists' campaign Informer n'est pas un délit (To inform is no crime) has called on President François Hollande to publicly support the whistleblowers.
Marine Le Pen, the leader of France's far-right National Front (FN), on Tuesday called on Hollande to "put pressure on Luxembourg" to protect them.
Le Monde newspaper has claimed that the Panama Papers show that two of her collaborators set up a "sophisticated offshore system" of tax dodging but she has insisted that the party will emerge with "head high and clean hands" from the accusations.
French Finance Minister Michel Sapin, who has drafted an anti-corruption bill, on Tuesday confirmed that it would protect whistleblowers.
EU, Luxembourg wake up to tax evasion
The defence has invited European Competition Commissioner Marghrete Vestager, who has investigated several companies, including Fiat and Amazon, to testify.
Her office said on Monday that she was considering the request.
Since the Luxleaks revelations the commission has published an analysis of multinationals' use of Europe as a tax haven and last year published a plan to harmonise EU members tax regimes and fight tax evasion.
A coalition of the Democratic Party, the Socialist Workers' Party and the Greens replaced Juncker's Christian Social People's Party-led government in 2013.
The number of tax rulings issued by Luxembourg declined to 715 in 2014 and limited banking secrecy in 2015.
The trial is expected to continue until 4 May.