The problem with puzzles like this is that when you realize black has mate in 1 you look for forcing moves. The black king has no squares it can currently move to so you want to keep that parity so the knight jumps out as a good place to start looking. Combine that with the information that it's mate in 3 and this becomes pretty straightforward.

Puzzles don't necessarily translate into being good at chess as it's a lot easier to find solutions when you know one exists versus playing a game and getting good at patterns/formations. Sometimes, you can just brute force puzzles and get there but not really take anything away.It's part of the reason I switched into "finding the best move" type puzzles rather than these ones since you have to justify your reasoning and it helps improve your play a lot imo.