This scenario is rather intriguing, as the chances of reaching this endgame by promoting a pawn to a knight are extremely unlikely. Thus, I perceive it more as a theoretical puzzle. It is well-established that two knights alone cannot achieve checkmate, but when a third knight enters the scene, the stalemate issue is effectively eliminated, making the checkmate relatively straightforward.
The strategy is elementary, actually. The attacking king will stay close to its defending counterpart to push him to the board edge, while the knights would help by cutting off the escape routes on the sides. Then, once the king is contained in the corner, the stronger side will simply calculate the knight moves to deliver the final blow without drawing by a stalemate accidentally.
Just observe the following endgame:
As previously mentioned, this article contains an element of humor, since encountering such a situation in an actual game is highly unlikely. Nevertheless, it remains beneficial to develop proficiency with knights, as their unique abilities can prove advantageous in various tactical scenarios.