This is likely what you are looking for if you searched for "Neo Geo 3DS CIA." This method creates a specific CIA file for one specific game . When installed, that game appears as its own icon on your 3DS home menu.
There are two primary ways to use CIA files for Neo Geo on a 3DS: neo geo 3ds cia
To play Neo Geo games on a Nintendo 3DS using CIA files, you generally have two main paths: installing a standalone emulator This is likely what you are looking for
Depending on your hardware (Old 3DS vs. New 3DS), your performance may vary. 3DS:Installing software - Hacks Guide Wiki neo geo 3ds cia
Why people use CIAs for Neo Geo