Hilchos Lashon Hara, chapter 2, section 3

Some say that if a person relates lashon hara before three people, even though the speaker certainly transgresses the prohibition against lashon hara, if one of the three goes and tells it to someone else, he will not transgress lashon hara, because since three people know about it, it is considered as if the whole world knows. This is only if he [one of the three] told his fellow incidentally, and not if he intends to reveal the story and denigrate him more.
Even if he does not say the story in the name of the one who told him, but simply says that so and so was said about so and so, nonetheless, he transgresses the prohibition against lashon hara.