You can select all unmerged changes from older to yours for merging into mine with the --ed (-e) option. You can select only the nonoverlapping unmerged changes with --easy-only (-3), and you can select only the overlapping changes with --overlap-only (-x).
The -e, -3 and -x options select only unmerged changes, i.e. changes where mine and yours differ; they ignore changes from older to yours where mine and yours are identical, because they assume that such changes have already been merged. If this assumption is not a safe one, you can use the --show-all (-A) option (see Marking Conflicts).
Here is the output of the command diff3 with each of these three options (see Sample diff3 Input, for the complete contents of the files). Notice that -e outputs the union of the disjoint sets of changes output by -3 and -x.
Output of ‘diff3 -e lao tzu tao’:
11a -- The Way of Lao-Tzu, tr. Wing-tsit Chan . 8c so we may see their result. .
Output of ‘diff3 -3 lao tzu tao’:
8c so we may see their result. .
Output of ‘diff3 -x lao tzu tao’:
11a -- The Way of Lao-Tzu, tr. Wing-tsit Chan .