Even if you manage to install Minecraft, I heqvily doubt it'll work (since you said on an o3DS), unless it's a port, and not the official one.

there is no official port for o3ds, but you can get the homebrew app "3dscraft"

craftus reloaded

I dont think its possible. Even if it is, it will be so laggy that it is umplayable. There is a "old 3ds mode" on my new 3ds and if i try to play minecraft with it, its just terrible.