Public knowledge is not lashon harah

 Chofetz Chaim (clall 2) goes into great detail to try and disprove the simple understanding of Bava Basra (39b)/Erachin (16a), Tosfos and the view of the Rambam. These sources seem to say that the essence of lashon harah is speaking gossip in a concealed manner - not just saying negative things about others. Thus if it is said in a way that it will become public knowledge then there is no prohibition of lashon harah. This is apparently closer to the peshat of Vayikra (19:16).

Erachin(16a): Disparaging remarks said in front of three people is not considered lashon harah. What is the reaon? Because your friend has a friend and your friend’s friend has a friend [and thus it is considered public knowledge].

Tosfos (Bava Basra 39b): Negative words about another person that are said n the presence of three people is not prohibited as lashon harah - either to say or to repeat it to others This is clarified by the statement said in Erachin (15b) There this statement is said in the context of R’ Yossi statement in which he said, “I have never said something and looked around to see who was listening.” That means he wasn’t concerned if the one he was talking about was there. [That proves that saying negative comments in the presence of three people is not considered lashon harah – Rashi].

Meiri(Shabbos 33b): Even though lashon harah is the foundation of a number of sins and something which our sages spend much effort in severely attacking – there are two conditions in which lashon harah is permitted 1) This is mentioned in Erchin (15b) that derogatory words which are said in the presence of the one being talked about are not considered lashon harah. The gemora questions this and says that lashon harah said in the presence of the one being criticized is surely impudence and lashon harah? This is explained with a statement of R’ Yose that he never said any words about another person and looked around to make sure the person wasn't there. In other words if the person speaking does not refrain from saying the negative words before the one he is speaking about – there is a leniency and this is not considered lashon harah. For example if the speaker directly complains about the other person or calls him a thief and others such negative statements in his presence – the speaker is not considered speaking lashon harah. That is because he wants the person he is criticizing to hear what he is saying. 2) This is also mentioned in Erchin (16a) that whatever is said in the presence of three people is not considered lashon harah. That is because the speaker is saying the negative things with the full desire that his views reach the person he is speaking about. That is because all matters that are known to three people is not considered concealed or a secret. Therefore the speaker is not considered to be saying lashon harah. 

Rambam(Hilchos De’os 7:5): Speaking lashon harah is prohibited whether the person being spoken about is present or not. Those words which will cause harm to another person - either physically or financially when repeated by others or even if it upset him or frightens him – is considered lashon harah. If these types of words are said in the presence of three people then it can be assumed that they have become public knowledge Therefore if one of the three repeats the negative words which he had heard on another occasion – there is no prohibition of lashon harah. However this is only permitted if he did not intend to spread the news and cause it to become more known.