Gemara

(R. Chiya bar Aba): Macha'ah must be in front of two.

(R. Avahu): Macha'ah must be in front of three.

Suggestion: R. Chiya bar Aba and R. Avahu argue about Rabah bar Rav Huna's law;

(Rabah bar Rav Huna): Lashon ha'Ra does not apply to anything said in front of three people.

R. Chiya bar Aba says that two suffice for Macha'ah, for he argues with Rabah. R. Avahu holds like Rabah, therefore, he requires three.

Rejection: No, all agree with Rabah. R. Avahu allows Macha'ah not in front of the Machazik. Three are needed to spread the word everywhere. R. Chiya require Macha'ah to be in front of the Machazik. Surely the Machazik will hear, so it suffices to tell two.

Erchin 15b (Rabah): It is Lashon ha'Ra to say "there is fire in Ploni's house." (This implies that they are gluttonous, and are always cooking.)

Objection (Abaye): One may tell people where to find fire! To say offensively 'where will you find fire, if not in Ploni's house?!' is Lashon ha'Ra.

.(Rabah): Anything said in front of the party discussed is not Lashon ha'Ra. This is like R. Yosi;

(R. Yosi): I never said something and turned around to see if the one I spoke about was there.

(Rabah bar Rav Huna): Lashon ha'Ra does not apply to anything said in front of three people.

This is because your friend has a friend, and he has a friend...

Sanhedrin 31a: Once, a matter was said in the Beis Medrash, and it became known that a certain Talmid revealed it 22 years later. R. Ami expelled him from the Beis Medrash, and announced 'this person reveals secrets.'

Rishonim

The Rif (Shabbos 14a) brings the Gemara in Erchin.

Ran (DH Leis): One who repeats it does not transgress, for the teller showed that he does not care if Ploni finds out, for in the end it will be revealed.

Rambam (Hilchos De'os 7:5): One may not speak Lashon ha'Ra about Ploni in front of him or in his absence. Lashon ha'Ra is saying things that, if they are heard through gossip, can cause bodily or monetary harm or anguish or fear. If they were said in front of three, it is already known, and if one of the three retells it, it is not Lashon ha'Ra, provided that he did not intend to reveal it more.

Kesef Mishneh: The Heter is only if he happened to mention the matter. He may not intend to spread it. The Rambam did not need to say that if the one who said it told the listeners not to reveal it that they may not do so, for we cannot say that word will spread.

Hagahos Maimoniyos (7): We learn from Sanhedrin that if the teller said not to reveal a matter, one may not reveal it even if was said in front of many.

Question: Perhaps R. Ami expelled the Talmid because the Talmid intended to spread the word!

Answer (Maharshal on Semag, Sof Lav 9): The Gemara says 'he revealed'. This does not connote intent to spread the matter.

Chafetz Chayim (Hilchos Lashon ha'Ra, Be'er Mayim Chayim 2:3): Perhaps the Rambam learns the Isur to intend to spread the matter from this case. Alternatively, he learns from the Yerushalmi. It proves that one may speak Lashon ha'Ra about those who make controversy.from Noson's directions to Bas Sheva to tell David about Adoniyahu. Even though it was famous and David was sure to find out, it would have been forbidden to intend to spread the word, if not for the Heter to speak about those who make controversy.

She'altos (Va'Yeshev 28, b'Sof): Normally, one needs permission to repeat anything he was told. Rabah bar Rav Huna teaches that if it was told in front of three, this is like permission.

Ha'Emek She'elah: Rashi (Erchin Reish 16a) explains similarly.

Tosfos (Erchin 15b DH Kol): Rabah bar Rav Huna refers to statements like 'where will you find fire...,' which can be said in a positive or negative way. Derogatory remarks are Lashon ha'Ra even in front of three.

Magen Avraham (156:2): This is because what is said in front of three is as if it was said in front of Ploni. (Surely, he says it in a positive way, for Ploni will find out.)

Hagahos Ashri (3:30): If one talks about Ploni in front of three, Lashon ha'Ra does not apply, for presumably he is not concerned if Ploni hears. One who hears may tell Ploni.

Rashbam (39b DH u'Man): My Rebbeyim say that Macha'ah is Lashon ha'Ra, to say that Ploni is stealing the land. This is why (in the Hava Amina) Rabah requires it to be in front of three, to permit telling others. This is wrong. Macha'ah is meant to reach the Machazik, so he will be careful with his document! It is a Mitzvah to tell others!

Tosfos (39b DH Leis (1)): We say that a loan without a document does not become known even if it was in front of many witnesses. This is because people borrow covertly. Also, the witnesses do not know who will buy from the borrower and needs to know about the loan. People know that the Machazik must hear about the Macha'ah.

Tosfos (39b DH Leis (2)): Something said in front of three is not Lashon ha'Ra for the one who said it, like it says in Erchin.

Poskim

Magen Avraham (156:2): If something derogatory was said in front of three, it is already known, and if one of the three tells it again, Lashon ha'Ra does not apply. This is if he does not intend to spread the word.

Chafetz Chayim (2:3): Some say that if Reuven spoke detriment about Shimon in front of three people, even though he surely transgressed Lashon ha'Ra, if one of the three who heard later tells others, he did not transgress, for surely the matter will become known, for your friend has a friend... and the Torah did not forbid something destined to be revealed. If he intends to spread it, even if he does not say from whom he heard it, just that he heard so about Shimon, he transgresses Lashon ha'Ra.

Chafetz Chayim (4): The Heter when he does not intend to spread it is only for those who heard it firsthand in front of three, but not for one (Yehudah) who heard from Levi, who heard from Reuven in front of three, even if he does not mention Reuven or Levi, It is permitted only if it is already know to everyone. This is not only if Yehudah does not know whether Reuven really said so, in which case he may not believe Levi that Reuven transgressed Lashon ha'Ra. Rather, even if he knows that Reuven spoke bad about Shimon, but he does not know whether it was in front of three, and Levi says that it was, Yehudah may not rely on Levi. Perhaps it was not said in front of three, and it is not prone to become known. Therefore, Yehudah may not tell anyone.

Chafetz Chaim (5): If a matter was told to three people who fear Hash-m and are careful about Lashon ha'Ra, the matter is not prone to be revealed, so one may not repeat it. The same applies if even one of them fears Hash-m, for then there are not three who will publicize it.

Chafetz Chayim (6): It seems that this Heter is only in the city in which iit was heard in front of three, but not in another city, even if caravans are frequent between them.

Chafetz Chaim (7): If Reuven told the listeners not to repeat the matter, even if it was told to many, Lashon ha'Ra applies even if one just happens to reveal it. This is even if he sees that one or two of the listeners did not obey and repeated it.

Chafetz Chayim (9,10): Surely, one may not retell more than he heard or embellish the matter, for this debases Shimon more than the initial story, and it shows that he accepted that the matter is true. All agree that one may not do so (he may only be concerned lest it is true). One may never tell bad things to one who will accept that it is true, and possibly retell it and add things, due to Lifnei Iver. One must stay far from this leniency, for it hardly ever applies, and also it requires investigation if the Halachah follows this opinion, for many Poskim hold that it has no source in the Gemara.