3-Up Moon is an item in Super Mario World and its several remakes and re-releases. If Mario or Luigi finds this item, it gives either one of them three extra lives. This is one of the rarest items in the game. 1-Up Mushrooms in comparison only rewards Mario and Luigi one extra life and are much easier to find.
New Super Mario Bros. U
This item is also found in New Super Mario Bros. U and its DLC New Super Luigi U. It is only found while in Boost Mode and even then, it is still fairly rare. The game tracks how many the players have collected.