3DS:Flash Cards

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

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 only able to run a ROM copied directly to a microSD card, it eventually got multigame support.

However, the Gateway is not technically a flashcard, but rather a novel accessory that fits in the gamecard slot and requires dedicated support from the OS; therefore, the Gateway requires the use of its dedicated and proprietary CFW, which has been criticized for including deliberate bricking code, lacking features compared to the competition, and taking a long time to receive updates (the final of which supporting up to the 10.2 operating system). To their credit, the Gateway CFW introduced the first practical cheat engine and EmuNAND implementation for the 3DS.

Many clones of the Gateway - all with even worse support and brick risk - existed, such as the MT-Card, Orange3DS, 3DS-Link, and R4i Gold 3DS Deluxe; curiously, even reputable brands later dabbled with this, namely the SuperCard DSTwo Plus and the EZ-Flash Redux.

Sky3DS
In late 2014 the first real flashcard for the 3DS was released, the Sky3DS (red button), accurately running clean dumps on unmodified consoles, at the cost of not having a menu and requiring the use of its button to cycle sequentially between ROMs. It also stores Card1 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 intrinsecally 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.