In the standard debian/ubuntu/mint repository, you can find a collection of useful tools for helping you with developing gtk3 apps. For me a big issue has always been finding icons to put on buttons. Install the following package:
sudo apt install gtk-3-examples
and run this command from a terminal:
gtk3-icon-browser
and you can find all the names of standard icons, including selectively seeing the prettier, simpler 'symbolic' set.