diff3

Compare three files

$ diff3 [path/to/file1] [path/to/file2] [path/to/file3]

$ diff3 -A [path/to/file1] [path/to/file2] [path/to/file3]

diff3 [options] myfile oldfile yourfile

diff3 compares three files line by line, showing differences and conflicts. It's typically used for three-way merges where one file is the common ancestor and two files are modified versions.
The output format is suitable for merge conflict resolution.

-A, --show-all

Show all changes, bracketing conflicts

Output ed script

Like -e but bracket conflicts

Output merged file

Align tabs properly

File order matters: mine, older, yours. Commonly used internally by version control systems. Part of GNU diffutils.

diff(1), merge(1), patch(1)