Laws of Lashon Hara / 10: 7-8 (76)

7. The authorization to speak about one who has wronged others, is only when the speaker has first rebuked him. If the speaker knows that he will not accept the rebuke, he is permitted to speak about him without first rebuking him but it must be done in front of three or more people, so that it will not seem to others that he’s going behind the sinners back, and trying to find favor in his eyes while secretly condemning him. Also, so that no one will suspect him of lying, and people are wont to lie in public.

8. However, if he fears that the sinner will harm him, he might be permitted to pass it on to less than three people.