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

A lot of GUI diff/merge tools have a 3 or 4 way merge view. I highly recommend Beyond Compare for resolving merge conflicts. Another (okay) tool is DiffMerge.

You can set up a custom mergetool to use with the git mergetool command. This is my .gitconfig configuration for Beyond Compare (Pro edition) and DiffMerge on my Windows machine using msysgit:

[merge]
    tool = bc3
[diff]
    tool = bc3
[difftool "dm"]
    cmd = C:/Program\\ Files/SourceGear/Common/DiffMerge/sgdm.exe \"$LOCAL\" \"$REMOTE\"
[difftool "bc3"]
    cmd = "\"c:/program files (x86)/beyond compare 3/bcomp.exe\" \"$LOCAL\" \"$REMOTE\""
[mergetool "bc3"]
    cmd = "\"c:/program files (x86)/beyond compare 3/bcomp.exe\" \"$LOCAL\" \"$REMOTE\" \"$BASE\" \"$MERGED\""

You can read more about various diff/merge tool configurations in the official Linux Kernel Git documentation for git config.