Shemiras Halashon Halachos

Hilchos Lashon Hara, chapter 2, section 7

The leniency of speaking lashon hara that was spoken before three only applies if the speaker spoke with no strings attached. But if he warned them to not reveal the matter further, it is forbidden to speak of it further. Even if he sees that one or two of the listeners did speak of it further despite the warning, it is still forbidden to speak of it to other people.

Hilchos Lashon Hara, chapter 2, section 8

Regarding the wording of the warning, it makes no difference whether he told them not to speak of the matter he spoke of, or that he warned them that the story should not become known. In any event it is forbidden to related the derogatory information to someone else, and how much more to the person the derogatory information was spoken of. The entire leniency of "before three" only applies where there were three listeners, but if two spoke before two, one may not speak of it further.