These formulae provide multiple versions of GTK+ 3 packages.
Just brew tap twstrike/gtk-versions and then brew install <formula>.
If the formula conflicts with one from Homebrew/homebrew or another tap, you can brew install twstrike/gtk-versions/<formula>.
You can also install via URL:
brew install https://raw.githubusercontent.com/twstrike/homebrew-gtk-versions/master/<formula>.rb
First, please run brew update and brew doctor.
Second, read the Troubleshooting Checklist.
If you don’t read these it will take us far longer to help you with your problem.
brew help, man brew or check our documentation.