Help-Debug shows: "GTK 2.24.31, GLib 2.50.3"
Yeah GTK2
Your package list shows that there is not a GTK3 package for Debian, thats why I said you need to compile to test it.
You will need the debian `build-essentials` package installed and the GTK3 `-dev` packages.
Download the Geany source tarball and extract it to a new empty directory, lets say `/some/where` so you get `/some/where/geany`.
``` cd /some/where/geany; ./configure --prefix=/some/where --enable-gtk3 --disable-html-docs; make install; cd ../bin; ./geany -c ../config ```
(well you said you wanted one command :)
That gives a completely isolated Geany in `/some/where` for testing that does not interfere with your installed one or your normal user config.