The Infernal Trio (1974) on Netflix

A cunning French legal adviser teams up with an unemployed German girl and her sister to commit murders in Marseille.

In 1930s Marseilles, the former World War I hero, Georges Sarret, now earns a living as a respectable lawyer. However, under threat of his German mistress Philomène Schmidt's imminent deportation, the calculative and ambitious advocate will soon hatch a plan so simple and outrageous that would catapult the decadent couple and Philomène's sister, Catherine, to riches. As a result, the infernal trio will entangle themselves in a game of fraudulent marriage, old-age insurance, and of course, murder, nevertheless, bold but sinful schemes rarely are fail-safe.

—Nick Riganas