Recent Git versions have built-in support for kdiff3, so there’s no need to configure it manually using the generic cmd and args settings. Instead do:
$ git config --global merge.tool kdiff3
And if kdiff3 is not in your PATH environment also do:
$ git config --global mergetool.kdiff3.path /Applications/kdiff3.app/Contents/MacOS/kdiff3
This makes git mergetool launch kdiff3. Note that there is no way to configure Git to automatically launch your merge tool after a manual merge that has conflicts.
In case you really want to see how Git is calling kdiff3 internally, take a look at the built-in mergetool configuration for kdiff3.
Edit: For Beyond Compare 4, which now also supports Mac OS X, simply exchange kdiff3 with bc3 (yes, “3”) and adjust the path in the above lines. Starting with Git 2.2.0 you’ll be able to use bc as an alias for bc3 so that you do not have to care about the version number.