Hilchos Lashon Hara, chapter 2, section 5

If lashon hara was said in front of three people, usually repeating it further does not break the prohibition of lashon hara.

But that is only with ordinary people. If the three are G-d-fearing people who are careful about lashon hara – or even if only one of them is like that – it is forbidden to repeat it, because it is not certain that the story will come out.

Also, if one of the three is a friend or relative of the person spoken about, it is forbidden to repeat it. A friend or relative will surely not spread the story, so as not to shame someone close to them.