Here is @Joseph's accepted answer, but with the default Mac install path location of kdiff3
(Note that you can copy and paste this and run it in one go)
git config --global --add merge.tool kdiff3
git config --global --add mergetool.kdiff3.path "/Applications/kdiff3.app/Contents/MacOS/kdiff3"
git config --global --add mergetool.kdiff3.trustExitCode false
git config --global --add diff.guitool kdiff3
git config --global --add difftool.kdiff3.path "/Applications/kdiff3.app/Contents/MacOS/kdiff3"
git config --global --add difftool.kdiff3.trustExitCode false