Two race marshalls and a photographer died in the accident, said police in the department of Gers.
The incident happened at around 1100 local time during course testing close to Laas-Tillac, 110 kilometres (70 miles) west of Toulouse.
The two race officials, a man and a woman, were in their sixties while the photographer was in his forties, police said.
According to preliminary investigations, the driver, who was unhurt in the crash, lost control of the car while volunteers were erecting security barriers.
