Laws of Lashon Hara / 10: 9-10 (77)

9. Although it is proper for one who tells lashon hara about wrongdoers to do so in front of three listeners, if the speaker is known to all as someone honest and truthful, he is permitted to pass it on to less than three people as he will not be suspected of lying or flattery.

10. The authorization to speak about a sinner includes both a sin committed between man and his fellow or between man and God. However, in the latter case, the speaker must have seen him intentionally commit the sin numerous times, and it must be recognized by all as a sin.