For what it’s worth, here’s how I solved it:
1) Run in Terminal:
sudo ln -s /Applications/Sublime\ Text.app/Contents/SharedSupport/bin/subl /usr/local/bin/subl
This adds a subl alias to /usr/local/bin/ pointing to Sublime Text 3 app’s binary file. Now running subl in Terminal will launch Sublime Text 3 app.
2) Run in Terminal:
git config --global core.editor "subl -n -w"
This adds editor = subl -n -w to the [core] section of the ~/.gitconfig file. Now running git commit in Terminal will launch Sublime Text 3 app (subl) in a new window (-n), and the command line will wait (-w) until the commit message is saved and closed.
Image of final workflow added in response to clarifying question in comments below:
Official Sublime Text 3 doc: http://www.sublimetext.com/docs/3/osx_command_line.html