Description of problem: Please add latest version of mingw64-libadwaita for cross-compilation of modern GTK applications. This might help with compilation flags, patches, etc.: https://packages.msys2.org/base/mingw-w64-libadwaita
As already discussed on fedora-mingw, the best way to make this happen is submitting a package for review. Also, this is not a bug related to mingw-gtk3.