The so-called "Bygmalion affair" concerns invoices for events staged by PR company Bygmalion during the campaign that were allegedly charged to Sarkozy's UMP party rather than the campaign coffers.
Former top aide Eric Cesari, described as the "eyes" of the former president, party financial director Fabienne Liadze and former communications chief Pierre Chassat were charged with forgery and abuse of trust, a legal source told AFP.
Sarkozy announced two weeks ago that he would run for the leadership of the conservative UMP, seen as clear positioning for a presidential bid in 2017.
According to a source close to the inquiry, the UMP picked up the tab for around 18.5 million euros (USD 23.3 million) in expenses that should have been billed to the Sarkozy campaign.