From the Super Mario Wiki
3-Up Moons are moons that first appeared in Super Mario World. They are crescent-shaped moons and have eyes and a smile, colored yellow, and can be found on the ground. They are extremely rare, and very hard to get. They have a similar effect as 1-Up Mushrooms; however they give Mario or Luigi three extra lives instead of one. New Super Mario Bros. 2 introduced Moon Coins, which are found in World Star; the icon on the coins are similar to the 3-Up Moon. In the Star World of Super Mario World, the moon seen in the sky resembles a 3-Up Moon.
Super Mario series
Super Mario World
The locations of the 3-Up Moons in Super Mario World are as follows. If the player replays the level, the moon will disappear from where it was until the game is reset. Most, but not all, 3-Up Moons are only accessible if Mario or Luigi as Caped Mario or Caped Luigi.
Super Mario Advance 4: Super Mario Bros. 3
Game and Watch Gallery 4
Appears in the "FIRE" mingame, they come out of Yoshi eggs and give multiple points when bounced.
New Super Mario Bros. U
3-Up Moons reappear in the Wii U game, New Super Mario Bros. U. They are within 3-Up Moon Blocks, a total of four 3-Up Moons can appear and only be collected with the efforts of the GamePad player and Wii Remote player(s).
In Mario Family, there is a sewing design of a 3-Up Moon.
The 3-Up Moon makes an appearance in volume 3 of the Super Mario-Kun. It serves a different purpose, powering-up Mario rather than granting him 3 extra lives. This allows Mario to defeat a Buzzy Beetle-like enemy.
Names in other languages