To get 3 stars you have to play a level and get more than 30% fish than the CPU competitor.
You have unlimited dynamite, so on the first level of Jiggy Jungle Cup just go crazy chucking dynamite everywhere and pick up the fish. Also use your special weapons using X when you come across large clusters of fish.
If you don't get it on the first level I wouldn't worry as this achievement will likely come early in the game as the CPU is not much competition early on.
Good luck!!