Shemiras Halashon Halachos

Hilchos Lashon Hara, chapter 2, section 9

Whenever it is permitted to relate lashon hara that was said before three, this only means that the repeating itself is not forbidden. But it is forbidden, heaven forefend, to add one word or make the story seem more believable to the listener, since he is causing more harm through his words than if the story had only become known through the logic that something related before three becomes publicized automatically. Also, when one makes the story more believable, it proves that he has accepted it as being true.

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.