Sefer Chofetz Chaim chapter 10 halachos 7, 8

 In a situation where one should not rebuke the person who speaks lashon hara about others because you don’t think they will listen to you, and in fact it might be better if you say nothing because if you would say something and they don’t listen to rebuke then they will lift up their defenses and might never change even if others will come later and rebuke him. At least if you tell others without rebuking him first, they might be able to get through to him at some point and he’ll understand and change his ways. However, when you meet all the prerequisites, and you weren’t able to rebuke him then you can only spread the word in most circumstances if you say it in front of at 3 people. This is for two reasons:

1. So you don’t look like you are flattering him by only warning others about his lashon hara in private but saying nothing about his bad ways in public.

2. So people won’t think you are lying since you are willing to say it in front of a group which will most likely lead to him hearing about what you said about him.

 So since you have the audacity to speak out about this lashon hara speaker in public saying how bad he is and you should not accept a word of what he says then you have more credibility. The one exception when one can tell 1 or 2 people at a time is if you are afraid of dangerous repercussions if the lashon hara speaker finds out you are talking about him. Them Chazal says you are allowed to flatter bad people in order to save your neck and if people know you are being secretive because you a red afraid of violent repercussions then they will still take what you say into consideration and not think you are lying. However, this does not mean those listening to the person speaking out against the perpetrator are allowed to accept as truth what was said even if it is permissible to say it. They can only listen, be concerned, take proper precautions, and investigate more into the matter to see if it is really true or not. This is because it possible a fact was left out or added into the story which might change everything so one can never believe indefectibly what others says even if willing to be said in public unless he questions and proves himself to be sure it is true.

 This is very important in today’s day and age where there is so much news being thrown out in public through the media and other sources which sound true and is broadcasted to millions of people but many times we find there is a fact or something missing or added to the story that changes everything so it’s important for is to have the attitude to not believe anything decisively without looking into the matter ourselves. Again, proper precautions are permitted but assuming what was heard is absolutely the truth and making judgments is not permitted until proven to be in fact true.