World 3 (New Super Mario Bros.)
From the Super Mario Wiki
World 3 is a world encountered by Mario and Luigi in the game New Super Mario Bros.. This world features a large beach-covered island, with many ocean and beach themed levels. This world has many Cheep Cheeps, and has the first Ghost House in the game. The boss of this world is Cheepskipper.
It is not essential for the player to complete this world. The player can travel to World 4, if they defeat Mummipokey from World 2 in mini form. After the completion of this world, the player instantly gains access to World 5.