Hilchos Lashon Hara chapter 10, section 7

Now we will explain the third condition from paragraph 2: that you must first correct the wrongdoer.

If you know he will not listen, you do not need to correct him. But then you must be careful to tell what happened in front of three people. That counts as saying it to his face, because three people will spread the story.

Why? If you speak in front of only one or two people, it looks like you do not want him to know you spoke about him. It looks like you are secretly putting him down, and enjoying it. Also, the one or two listeners will suspect the story is not true, since you did not correct him before speaking to them. In that case, nothing good will come from it.

However, the listeners may not assume that what you said is true and put him down, as explained above (7:1). Even if the story is true, perhaps some detail is missing that changes the whole picture.

But the listeners may suspect that it is true, and correct the wrongdoer. He may listen to them. And there may be other benefits, as explained above in paragraph 4.