The default plugins included with Sublime Text are a great source of information on how to use the Sublime Text 3 api. Unfortunately, the default plugins can’t be found in the usual Packages directory. On OSX, they are neatly tucked away in the application installation file located at:
/Applications/Sublime Text 3.app/Contents/MacOS/Packages
The default plugins (and configuration) are stored within a file called: Default.sublime-package, which is simply a zip file.
To view the files you can extract the Default.sublime-package to a temporary directory with unzip:
unzip /Applications/Sublime Text 3.app/Contents/MacOS/Packages/Default.sublime-package -d some/temp/dir
And now you have full access to a host of example plugins to work with.