I already blogged about this previously and presented the different approaches - SEE HERE.
Reading the gemara simply, it seems to imply like the approach of the Rambam that once loshon hora was violated in the presence of 3 people, it can be assumed that it is already public info (even though the way it becomes public info is through an issur being violated) and can be said over by the 3 who heard it.
Being that the heter is based on the assumption that the word will get out, the Chofetz Chaim (perek 2) has many stipulations as to when it will be permitted:
1. It can be told over casually but not with the intention of spreading the news further. The Yad Ketana holds that even when said casually, it can only be said if that was the topic of conversation, but one cannot move the conversation in that direction in order to share the loshon hora.
2. It cannot be repeated back to the one it was said about. Although the Rashbam permits even this and doesn't consider it to be a violation of rechilus, the chofetz chaim isn't willing to pasken like that.
3. Only one who was part of the group of 3 can say it over, but not the person who heard it from him until it has become well known information.
4. If any of the group of 3 are exceptional yarei shamayim who wouldn't repeat it over, it cannot be repeated over by anyone in the group. He learns this from the gemara in sanhedrin 29a that considers the judges revealing their votes to be loshon hora even though the 3 judges know about it. He suggests that if any of the 3 wouldn't repeat it over because they are the subjects friends or family, it also cannot be repeated.
5. It can only be repeated to places where the news would have spread such as within the city. Mostly likely this condition no longer applies since information spreads very quickly to the 4 corners of the world.
6. If the original person who told it over warned the 3 listeners not to say it, one cannot assume that it will spread and cannot repeat it.
7. One cannot embellish the story in any way. It has to be said exactly as it was heard.
8. The heter is only for the one saying it over, but the listener still cannot believe it as factual. Therefore if the one telling it over knows the nature of the listener and that he would accept it as fact, he cannot tell it to him.