Hilchos Lashon Hara, chapter 2, section 4

The permission to repeat something that was said in front of three only applies to one of the three who heard it. Someone who heard it from one of them (that it was said in front of three) may not repeat it – unless the story has anyway already become known to everyone.

And of course, if that person does not know on their own that the story is true, they certainly must not believe the one who says they heard it in front of three.

But even if a person knows that Reuven said something bad about Shimon, but does not know whether it was said in front of three or not, they may not repeat it. The worry is that it was not said in front of three, and so it will not come out on its own.