Sefer Chofetz Chaim Chapter 2 footnote 9 and Halacha 5 with footnote 12 & 13

Footnote 9: Just to be aware of the severity of lashon hara. The starting assumption is that lashon hara or rechilus is ischazik issura, meaning that it is halachically assumed to be forbidden like any other prohibition which can’t just be permitted based a aingke person’s hearsay and that is why the person who lashon hara was repeated to after it was heard in front of three can’t just repeat it again since a single witness isn’t believed on assumed prohibition unless he can prove that he can make it permissible which is virtually impossible by lashon hara especially since meeting all the parameter of the permissibility of apei tlasa is very rare.

Halacha 5: If the 3 people in the group of apei tlasa are G-D fearing people who are careful not to speak lashon hara then The permissibility of api tlasa doesn’t apply because the word won’t spread. This means that the whole permissibility is based on the fact that non-G-D fearing people talk and rumors then spread whether true or not. It so happens that it makes no difference whether all 3 who are listening are G-D fearing or even just one, or if one of them was just a relative or a good friend of the one being talked about then we have to assume the lashon hara wouldn’t spread because there aren’t at least 3 people who might leak it to anyone else.

 Footnote 12 says: The source of this Halacha comes from a Mishna in the first chapter of Sanhedrin that a judge, presumably a G-D fearing individual cannot report to anyone that he found the litigant innocent and the other 2 judges found him guilty. Even if it was 7 judges and 4 said guilty and 3 said innocent so now there are 3 people, apei tlasa, who heard the four judges say guilty still the 3 can’t spread the news of which judges gave the guilty verdict, it appears from the Mishna therefore it must be that apei tlasa doesn’t apply to G-D fearing Jews. He brings many proofs to prove his reading of that Mishna.

Footnote 13: There is one exception to the circumstance where one of the 3 is a relative, friend, or G-D fearing which is a case of a protest when an owner of a property claims to them that so-and-so is poaching on my land land which is ok to believe and spread so that the so called poacher can be sure he keeps the appropriate documents to prove he is allowed on the land. So even a relative, friend , or G-D fearing Jew would spread the word just to help him not to get into trouble.