Using CIA and/or 3DS files on Nintendo 3DS

I recently got a N3DS. After some research, I found out there are 2 kind of roms to play on a 3DS: .3DS and .cia.
The 3DS is for games on a cartridge, while the cia files are games bought on the eshop. But now is my question: how do I play those games? Can I install them on the sd card, and play them, or do I need a flash card? And if I need a flash card, which one is the best one (recommended)? And can I use an old flash card of a nds on the 3DS? And can I update it to play 3DS games?
Gr. and thx.
Answer #1
I don’t know if you’re still in search of the information but for anyone else finding this for info:
There are 3 files for rom, .cia is so far the safest from NNID bans since they don’t have headers which are unique to games from cartridges. All you need to play .cia games is an SD card formatted to FAT32 and to set up a custom firmware such as rxTools or the like. I use rxTools however others use a cfw that works with arm9loader which boots up more reliably and can be unbricked without a hardmod. I’d suggest searching the gbatemp website for a guide to install cfw if you’re new. Afterward you transfer the .cia to the sd card and use a cia installer to install the game into the emunand on the sd card, then you can delete the .cia from the sd card btw. At the time of writing this there have been no reports of someone getting banned from online use while using only .cia roms
For .3ds and .3dz files you need a flashcart specifically made to emulate 3ds games, an old R4 or DStwo will be able to emulate NDS games but not 3ds games. Gateway, Mountain card, DStwo plus and Sky3ds plus are some viable ones and they cost upwards of $60. Gateway and Sky3ds plus are the top contenders, gateway being the oldest and works by running cfw from the card(s) (there are two, one is needed to launch the exploit to run the gateway cfw then the card is switched to the other one which contains the roms). Sky3ds/ sky3ds plus work by imitating the cartridge of an actual game and has buttons on it to switch which one shows up in the official firmware menu. the old ones were permanently limited to certain number of games that couldn’t be removed but that was fixed with plus. This one has online capabilities and was considered the best for a while until nintendo started banning users who had the same headers in their .3ds and .3dz files, making the sky3ds users the most affected. Private headers are needed to be obtained from personal games and put into the .3ds files to be safer but it’s unknown whether this will last. It’s currently unknown how Nintendo identifies pirated games but the theory is that multiple people playing with the same public header (ie. people who downloaded the same exact .3ds/.3dz rom without modifying it) at the same time are seen and they both get banned since the headers are supposed to be unique.
As a warning, do not get the DStwoplus to play 3ds games. The DStwo is one of the best DS flashcarts for DS and slot 2 emulation and the DStwoplus improves this so is great for DS GB GBA and SNES games, but the team stole the Gateway code used for 3ds emulation. Ironically, Gateway doesn’t like being on the receiving end of pirates and has purposely put in code to brick a 3ds if gateway code is being run on none-gateway hardware, there was a massive wave of bricking from this a few years back and recently more threats were made.
From my biased opinions; if you like installing stuff yourself and saving money at the expense of simplicity you should get a large SD card and instal custom firmware by following a guide from gbatemp. If you prefer to pay for a hassle free experience and not worry about setting things up and maintaining cfw against nintendo updates then you should get a Gateway card.