It's an Open Source software which supports building for Linux, MacOS and Windows.
Pre-built executables/applications can be downloaded at: https://github.com/Lorenzooone/cc3dsfs/releases/latest.
The software has many features, like: Cropping, PAR, Screens Relative Positions, Individual Screens Rotation, personalizable Profiles, and many more. A README file is included with all releases for instructions.
It supports both a regular windowed mode for recording via OBS, as well as a Fullscreen mode to actually play your games on a monitor/TV.
With main releases (like this one), binaries for Raspberry Pi-specific builds will be included, with support for GPIO buttons.
A setup zip to make it run on Raspberry Pi in kiosk mode is also included, with instructions for the installation.
For now, it works with N3DS XL, DS and older 3DS boards. It also supports IS Nitro devices.
Enjoy!
Below some examples of what you can achieve with it in fullscreen mode (Though let me remind you that it also supports the regular windowed mode for recording).
Spoiler
Open in a new tab if they look squished.
Pokémon X, a regular 3DS game. Pokémon Mystery Dungeon: Explorers of Sky, a DS game which has the main gameplay on the bottom screen. Chrono Trigger DS, with the Top Screen's aspect ratio fixed to look more like it did on SNES.