There are checkmating positions. However, it is impossible to do so unless your opponent plays willingly.
two knights and king v. king?

you cant force blacks king to the corner so you cant checkmate them with just two knights and king vs king

In some cases, where the defending side has a pawn, you can force mate.
But for me it's very tough, always have to look in the book : (
I heard dat two knights and a king dont have power to chekmate. Here is an example of two knights and a king checkmating