Faience: http://tiheum.deviantart.com/art/GTK3-Gnome-Shell-Faience-255097456. This is a GTK 3 theme that will make your Gnome Shell desktop look exquisite. This has awesome brushed aluminium styled switches for controls in the GTK windows. This is one theme that you must install.
Hope GTK 3: http://grvrulz.deviantart.com/art/Hope-gtk3-206207315. Another quality GTK 3 theme for Gnome Shell. Well worth installing.
Gaia GTK 3 theme: http://half-left.deviantart.com/art/GNOME-Shell-Gaia-207574700. This is a green tinted theme; this would suit a Linux Mint machine perfectly.
Awance GTK 3 theme: http://thedeviantmars.deviantart.com/art/Adwance-gtk3-207704533. A brownish Ubuntu 8.10 styled theme. This would obviously suit an Ubuntu computer perfectly.
Mist theme for GTK 3: http://gnome-look.org/content/show.php/Mist-Redmond?content=155580&PHPSESSID=f5b8516a2b1f6f69a3ae2e28bd13d7e6. The Mist theme lives again for GTK 3.
MediterraneanNight Series 2.02: http://gnome-look.org/content/show.php/MediterraneanNight+Series?content=156782. A very highly rated theme for GTK 3. This would make any Linux machine look awesome.
Ambience Chrunchy: http://gnome-look.org/content/show.php/Ambiance+Crunchy?content=151181. The Ubuntu Unity Ambience theme ported to GTK 3 and Gnome Shell. This is another theme that would suit an Ubuntu machine if you are a fan of the default Unity look.