Knights always tell the truth,liars always lie,and knaves alternate between truths and lies.
Suppose you ask a native,"Are you a knight,a liar,or a knave?"
The native answers,and you are able to figure out what he is.
What is the native?
The native is a knave.
There are three possible answers the native could give to the question:
1) If he answers "knight", then the native could be a knight telling the truth, a knave telling a lie, or a liar telling a lie.
2) If he answers "knave", then the native couldn't be a knight, but could be a knave telling the truth or a liar telling a lie.
3) If he answers "liar", then the native couldn't be a knight or a liar, but could be a knave telling a lie.
Thus, since only one answer would enable you to figure out for sure the native's type, the native must have answered "a liar" and therefore is a knave.