This is a bit complicated, the way KDL3 achieves translucency is to run at double the display width of most other games (512 wide instead of 256 wide), and then foreground objects are shown every second column on the screen. On an old TV, the columns would blur together and you'd get a translucent effect, even though the game was just using (alternating on-off) opaque foreground layers.
To do this on 3DS would require at a minimum some kind of video filtering on the image to blur every second column together. Hopefully this is something
@bubble2k16 is interested in implementing, but it's a trick that very few games use (Kirby is the only one I know, personally), so it may not be a priority. It's certainly something the 3DS would be capable of, however, and the filtering should be doable on the GPU for additional savings.