Install command line tools:
then run this:
git config --global diff.tool bc3
git config --global difftool.bc3 trustExitCode true
git config --global merge.tool bc3
git config --global mergetool.bc3 trustExitCode true
ln -s /usr/local/bin/bcomp /usr/local/bin/bc3
now use git difftool and git mergetool.
For diff:
Visual Diff Tool: Other, Command: /usr/local/bin/bcomp, Arguments: $LOCAL $REMOTE
For merge:
Visual Diff Tool: Other, Command: /usr/local/bin/bcomp, Arguments: $LOCAL $REMOTE $BASE -mergeoutput=$MERGED

hi @jfromaniello
my bad, i did not reset merge