three-leaf clover
Refers to a clover with three leaves, often contrasted with the four-leaf clover (四つ葉のクローバー). Commonly used in everyday contexts and children's stories.
I found many three-leaf clovers in the park.
A three-leaf clover is not a symbol of luck, but a four-leaf clover is.
Compound of 三つ葉 (みつば, 'three leaves') + の (possessive particle) + クローバー ('clover'). The word クローバー is borrowed from English 'clover'.