It's really only one generation of backwards compatibility. The GBA SP is the last one that can play GBC games, as it's the last one with the 8-bit coprocessor - the DS can play GBA games because it's "small" processor is the same, but it can no longer play GBC games. The 3DS can no longer play GBA games as they no longer physically fit.
The 3DS can play DS and GBA games natively. While the lack of a GBA cartridge slot is a hindrance, it’s nothing that ROM injections on a hacked console can’t get around.
That along with the capability to emulate up to SNES games makes my 3DS something that still makes its way into my travel backpack. I leave the switch and GBA SP at home. They’re too big for what they offer.
