3. Expertise is required. Before us are three natives of the country of knights and liars $\mathcal{A}, B$ and $C$. $\mathcal{A}$ says: «We are all liars». $B$ says: «Exactly one of us is a liar». Who is $C$ - a knight or a liar? Can we determine who $B$ is?
3. $\mathcal{A}$ is a liar, as his statement “We are all liars” cannot be true. From B's statement: “Exactly one of us is a liar,” if it is true, it follows that $B$ and $C$ are both knights. If it is false, then $C$ is the only knight, since $\mathcal{A}$'s statement is false. Therefore, $\mathcal{A}$ is a liar, $C$ is a knight, and whether $B$ is a knight or a liar cannot be determined.