Hilchos Lashon Hara, chapter 2, section 10
Know also, that the whole leniency of something said before three only regards the speaker. But regarding the listener, if one knows that the listener will accept the story as true and perhaps add more to make it worse, it is forbidden to relate any hint of denigration of one’s fellow to such a person in every event. If one speaks to such a person one will transgress, “Do not place a stumbling block before the blind.” When we say that one may not speak further, this applies even if one says he heard such and such about so and so, without mentioning the name of the first person who spoke. Taking all these details we said into consideration, we see how one should keep far away from this leniency of lashon hara related before three, because there is almost no instance in real life where all these details would be fulfilled.