Shemiras Halashon Halachos

Hilchos Lashon Hara, chapter 2, section 1

One may not speak lashon hara against one's fellow even if the statements are correct, and even before a single person, and the more listeners, the greater the speaker's sin, since his fellow is denigrated more when his denigration is publicized to more people, and also, because he makes more people stumble in hearing lashon hara.

Hilchos Lashon Hara, chapter 2, section 2

When we find that Chazal permit lashon hara "before three," this relates to saying something that is not an outright denigration and is ambiguous. Something like this depends on the speaker's tone of voice when he talks. If he relates something like this before three people it is permitted, because since he knows that his words will certainly reach the ears of the person spoken of, he speaks in a positive way.