Huh? You can install games in .cia format. Every Virtual console game (NES, SNES if New3DS, GB, GBC) and GBA injects. You can also install 3DS games this way, and they work. You only need a flashcart if you want DS games.
Or you can use an app Freeshop to download nearly every game in the eShop directly to your console.
If you followed 3ds.guide, you should have ended with the FBI app installed, which helps you for installing apps and games from .cia files. Just look for the Freeshop CIA and install it with FBI.