You can buy the game directly from Nintendo's eShop on the Wii U or Nintendo Switch. For the Switch, look for "Super Mario 3D World + Bowser's Fury," which includes the base game and new content.
Direct links to download Super Mario 3D World ROMs are not provided here, as the distribution of copyrighted game files without authorization is illegal and violates Nintendo's intellectual property policies super mario 3d world rom download link
While ROM downloads can be tempting, there are risks involved: You can buy the game directly from Nintendo's