3DS:Flash Cards

The mostly questionable products passed as flashcards for the 3DS can be grouped as follows:

Vaporware
As early as September 2011, seemingly promising research by the Crown3DS team attempted to produce a viable gamecard emulation device, but nothing came out of it.

DS flashcards
As for the DSi, many 3DS-compatible DS flashcards used suggestive and almost misleading names.

Nintendo continued trying to block them until the 7.0 update: anything supporting this 3DS version will work on any DS-compatible console known to date (unless it self-destructs as not rare with many bottom-barrel products).

Some CFWs include patches to unlock any flashcard with a valid header (i.e. not the original autobooting DS/Lite only Slot-1 cards, like the R4) and even those can be used with a forwarder such as NTR Launcher.

Gateway
By the end of August 2013, the first commercial backup loader for the 3DS had been released: the Gateway 3DS, which much later was revealed to be a product of the post-sellout Team Xecuter (who also made the StarGate 3DS, the TrueBlue Mini, the Classic2Magic, and the SX modchips).

Initially, it was only capable of running one ROM onto a microSD card, later, it eventually got multigame support for multiple ROM files.

The Gateway, however, is not technically a flashcard; rather, it's a novel accessory that fits into the gamecard slot (slot-1) and requires dedicated support for the OS; therefore, the Gateway requires the use of its dedicated and proprietary CFW, which has been criticised since the release of Omega, starting from version 2.0b2, where it included deliberate bricking code in their response to deter the use of clone Gateway carts that came out shortly after release. Not only did it brick consoles with fake Gateway carts, but also users of genuine carts. The CFW lacked features compared to their competitors and was taking a long time to receive updates (the last version supported up to the 10.2 system firmware). To their credit, the Gateway CFW introduced the first practical cheat engine and EmuNAND implementation for the 3DS.

Many clones of the Gateway have even worse support and a potential risk of bricks; many existed, such as the MT-Card, Orange 3DS, 3DS-Link, and R4i Gold 3DS Deluxe; curiously enough, even reputable brands later dabbled with this, namely the SuperCard DSTWO+ and the EZ-Flash Redux.

Sky3DS
In late 2014, the first real-time flashcard for the 3DS was released, the Sky3DS (red button), which accurately ran clean dumps on unmodified consoles at the cost of not having a menu or modifying the console with an exploit, requiring the use of its button to cycle sequentially between ROMs. It also stores slot-1 saves in internal memory, which cannot be cleared and only supports 10 such games ever; tacky workarounds exist, but the ultimate solution for this non-updateable card was to sell a blue-button model without this restriction.

But both models are incompatible with most 9.6+ titles using 0x1FE protection; this limitation too received both a workaround (editing the ROM, which breaks its signature and therefore requires a CFW to allow it again in the first place) and a new flashcard, the orange-button Sky3DS Plus, which also added a backwards-cycling button and dropped the requirement for the proprietary PC software in favor of taking clean ROMs and a configuration file on a standard filesystem.

Stargate
By the end of 2017, the Gateway team (launched under false name) a new product, the Stargate 3DS, effectively combining a DS and 3DS flashcard: its two buttons can be used to cycle between 3DS ROMs like on a Sky3DS+, but if pressed simultaneously, the card switches to operating as a DS flashcard, which, being almost intrinsically capable of running homebrew, includes in its kernel/menu the ability to directly select a 3DS title.

Unfortunately for the team, this premium product was too little (except in price) and too late, as CFWs with patches giving near total freedom in unsigned title installation and operation were already generally appreciated as superior, and both sighax and ntrboot were released in that summer, ultimately giving the ability to cleanly load CFW regardless of system version.