Okay, definitely not installed then.  Smallish packages with minimal dependencies
that use GTK3 would include mousepad, xarchiver, usbview, putty, and xdaliclock.
(Note:  I don't have all of these installed, I just queried for all gtk+-dependent
packages and checked a few that I thought were likely to have short dep lists.)

Also, if you happen to have inkscape, audacity, or handbrake installed,
they all use GTK3.  As do a lot of the closed-source chat programs (discord, skype,
signal, slack, whatsapp), although they may have skinned it to varying extents
that make it difficult to use to test anything.

E. Liddell

Felix, you can use the "urpmf" command to search package containing a specific file:

$ sudo urpmf /usr/bin/gtk3-demo                                                                                                       
lib64gtk+3.0-devel:/usr/bin/gtk3-demo
lib64gtk+3.0-devel:/usr/bin/gtk3-demo-application

In this case, the binary you are looking for is in 'lib64gtk+3.0-devel' package. Again, you will have several dependencies if you install this :)

François