Hilchos Lashon Hara chapter 10, section 7

We will now discuss the third condition we wrote in paragraph 2, that one must first rebuke the perpetrator. If one knows he will not listen one does not need to rebuke him. However, one must be careful to relate what happened in front of three people, which counts as speaking in front of the perpetrator [as three people spread the story]. Because if one speaks in front of one or two people it looks as if one doesn’t want the perpetrator to know one spoke of him and is trying to deceive him and disparage him secretly, and as if one is enjoying speaking of him. Also, the one or two listeners will suspect the story is not true as he did not rebuke the perpetrator before speaking to them and in such a case no benefit will result from speaking of him.
However, the listeners may not assume that what one said is true and disparage the perpetrator as explained above (7:1). Because even if the story one said is true, perhaps some detail is missing that completely alters the story. However, the listeners may suspect it is true and rebuke the perpetrator who may listen to them, and there may be other benefits as explained above in paragraph 4.