How can I see a “three way diff” for a Git merge conflict?

From git-merge(1),

An alternative style can be used by setting the “merge.conflictstyle” configuration variable to “diff3”.

In addition to the <<<<<<<, =======, and >>>>>>> markers, it uses another ||||||| marker that is followed by the original text. … You can sometimes come up with a better resolution by viewing the original.

This can be enabled using

git config --global merge.conflictstyle diff3

or right in ~/.gitconfigfile

[merge]
  conflictstyle = diff3