From the Super Mario Wiki
World 3 is the third normal world appearing in New Super Mario Bros. 2. This world is both beach and jungle-themed, although the music is similar to that of World 4 of New Super Mario Bros. Wii, which is a beach-themed level. The world is also similar to World 3 from New Super Mario Bros.. This world is unlocked after completing World 2-Castle or World Mushroom-Castle. The first half of the map is located at the seaside while the second half is located in land, where the jungle is, although the levels can take place between any of the two locations regardless of it's position on the world map.
There is a total of 12 levels and three Toad Houses (one of each kind) in this world, making it the longest world in New Super Mario Bros. 2, tied with World 4. Of the 12 levels, eight are levels on the main path. Of these levels, five are normal levels, one is a Ghost House and the other two are the Tower (or Fortress) and Castle levels. Of the remaining four levels, two of them are normal levels (one is a secret level), one is the Cannon level and the last is the Rainbow level. The world's boss is Wendy O. Koopa, who is fought at the Castle. However, the player can avoid fighting the bosses in this world by taking the world's Cannon, leading to World Flower.