Laws of Lashon Hara / 2: 7-8 (32)

7. Even if something is said over to three people but the speaker warns them not to pass it on, it is forbidden to pass it on. Even if one or two of the listeners did go and pass it on, it’s still forbidden for the third one to do so, even when his intention is not to publicize the matter.

8. This authorization of ‘something told to three’ does not apply when two people each pass it on to another two listeners. It only applies to one person who passes it on to three or more people.