World 3 (New Super Mario Bros.)
From the Super Mario Wiki
World 3 is third main world 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 has a total of eleven exits.
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.