if $N_3$ were a knight, then there are two knaves and a knight, but that would mean that $N_2$ tells the truth and hence he is a knight, which means that there are at least two knight or at most one knave. A contradiction. Hence $N_3$ is a knave. Clearly, $N_1$ is also a knave, for if he were a knight, then everybody would be a knave. A contradiction. Now as the first is a knave, then he lies, and hence not all of them are knaves and hence $N_2$ is a knight. Indeed, what $N_2$ says is consistent.