From the rules:
"The aim of the game is to collect the most fish.
You have to try to gather kitten gangs! Whoever manages to
gather either three identical cats or any three cats on the same colored background, receives a fish as a reward.

Uncover any card and place it in front of you. One by one all
players uncover a card to try to collect a kitten gang of three identical cats or any three cats on the same colored background.

- There can never be more than three cards in front of you.
When it's your turn and you already have three cards, you
have to return one of them face up to the center of the table
and only then can you uncover a new card.

- Instead of uncovering a hidden card you can also take a card
from the ones face-up in the center, except of course for the
one you just placed there.