About 200 people are to testify before three judges in the trial, which is expected to conclude May 16.
The three accused - Gustavo Lasi, 27, Daniel Vilte, 28, and Santos Vera, 34 - were led into the courtroom in handcuffs to hear the charges against them.
They listened impassively as the mother of one of the victims, 29-year-old Cassandre Bouvier, spoke about her daughter in a barely audible voice, asking that "justice be done."
"She would have been 32. But her path was crossed by the wickedness, the barbarism, the monstrousness that humanity is capable of," said Helen Kottak.
Bouvier and Houria Moumni, 24, were last seen as they entered La Quebrada de San Lorenzo park overlooking Salta in the afternoon of July 15, 2011.
They were found shot to death July 29, their partially clad bodies dumped near a scenic overlook in the park.
Lasi, a former city employee and occasional guide at the park, is the primary suspect.
DNA tests linked him to the women's rape.
He has acknowledged sexually assaulting them, but insists he did not kill them, blaming the other two suspects for their murders.
Vilte, a bricklayer, and Vera, a groundskeeper, deny any connection to the crime.
The three have been in preventative detention since 2011.