From the Super Mario Wiki
Green Coins are a variety of Coins that first appear in New Super Mario Bros. U and New Super Luigi 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.
Green Coins reappear in Super Mario 3D World. This time, they resemble small Star Coins, and eight of them are produced whenever the player runs or jumps through a Green Ring. A Green Star appears once all eight coins are collected.
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