From the Super Mario Wiki
Green Coins are a variety of Coins that first appear in New Super Mario Bros. U. They serve the same purpose as Red Coins, but these coins disappear much faster and only five groups of three are collected. They always appear in groups of three together forming a clover shape.
They rotate in the air but the effect they have is the same as that of Red Coins, as collecting them all gives the player a Power Up or 1-Up Mushroom. They are activated by moving through a Green Ring.