Necroposting, I know, but just in case someone else googles up this thread as I did.
The MPI3508 3.5" screen is cheap and sold a ton apparently.
So the problem is, the display is actually 480x320 but identifies itself as 1024x768 (!), which is not correct as size and not even as aspect ratio (4:3 vs. actual 3:2)
Trying to force 480x320 in armbianEnv.txt does not work, the display shows nothing; probably there are no proper timings in the kernel/DTB/whatever for such low an HDMI resolution.
So the trick is using 720x480, which is 1.5x the actual resolution. The aspect ratio is correct, the timings are there, and after all it's even quite readable.
So in /boot/armbianEnv.txt add:
extraargs=video=HDMI-A-1:720x480@60
disp_mode=720x480
and then recompile boot.scr:
mkimage -C none -A arm -T script -d /boot/boot.cmd /boot/boot.scr
