Let's summarize.
Nintendo is famously aggressive about protecting its intellectual property. Super Mario 3D Land is still a copyrighted title. Distributing or downloading the ROM without owning a physical copy is a violation of the Digital Millennium Copyright Act (DMCA) in the US and similar laws worldwide. While individuals are rarely sued, the websites hosting these ROMs are frequently shut down. Super Mario 3d Land 3ds Rom Download
With the 3DS out of production and digital purchases no longer available via the official store, many gamers have turned to emulation. Emulation itself is legal; it is the act of downloading ROMs (copyrighted game data) from the internet that exists in a legal grey area. Let's summarize