I think a forgotten ldconfig command in this case is not the reason that gtk3 libs are not found though it is always a good idea. As I understand gtk and co. test existance of packages not by linking to them but by looking for the corresponding pkg-config file.
If you simply run configure from the official pcakages, your libs will be installded under /usr/local whereas the system looks for pkg-config files only under /usr . There are two possibilities to overcome this:
1) set the environment variable PKG_CONFIG_PATH to point to the appropriate directory under /usr/local , e.g.
/usr/local/lib/pkg-config and run configure again
or, strongly recommended
2) get the slackbuildscripts for gtk3 and gtkmm3 from www.slackbuilds.org
for your slackware version and build
your own packages, which you can install afterward