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.