Serious as Pleasure (1975)

Original title: Sérieux comme le plaisir

"In Serieux comme le plaisir, two men and a woman live quite happily together in a romantic liaison. The woman is probably wealthy anyway, so the trio doesn't worry much about money. One day they decide to take a trip in their beat-up car, managing the whole affair in their own special, insouciant manner. They are followed by a suspicious policeman who thinks there's something fishy about this group..."