MATE, the Gnome 2 fork is still using Gtk3, although I think they have plans to migrate to Gtk3 in the future. Currently, is it possible to run Gtk3 applications in MATE?
MATE was forked on purpose to stay with Gtk2, for reasons of performance and maintainability; a desktop with similar panel arrangements but using Gtk3 is Cinnamon. See Why was MATE created on the Linux Mint forums.
When running Gtk3 applications in MATE you'll notice they look different, unless you are using a theme that supports both Gtk2 and Gtk3, with sensibly a similar look. You'll have to select the theme in both Preferences/Aspect applet for Gtk2 and gnome-tweak-tool for Gtk3.