You can find chicken as events and tournament rewards.

Without them, you should use uncommon heroes as food.

Hello, thanks for reply, i know i can use The uncommon Champs for food, but i need 3 stars champ to upgrade 3 rare Champs, idk what Champs to use for upgrade them on 4*

Any common is good, they are hardly of use not even in the beginning. In the beginning some uncommons can be handy. You of course will need to level and rank those commons up to rank 3

Thanks m8!