Ok, lets do this again.
In A9LH context you can't execute 3dsx files.
Why you say? 3dsx homebrew relies on libctru. The homebrew Launcher is a libctru based homebrew. Libctru uses svc for low level stuff. The svc subsystem is majorly in the arm11 kernel and the firmware (firmware as in firmware.bin). Guess what. Both are not ready and off in a9lh context. If you start the firmware yourself, well you got to boot the os.
So, it's possible to write a homebrew Launcher for a9lh? Yes it is. And it will able to launch 3dsx? No it wont. You'll have to either reinstantiate the svc subsystem (a hell) or manage communication between arm9 and arm11 by hand (a pointer hell), if and only if you got the arm11 cpu to init with some sort of custom arm11loader (arm assembly hell).
TL;DR: you can't execute 3dsx in a9lh. Also a9lh is not meant for this.
EDIT: But the other way around... (a9lh payloads in the regular homebrew launcher) That's another story.