If you have three versions of a file that you want to compare at once, use the diff3 command as follows:
$ diff3 file1 file2 file3 |
diff3 compares three versions of a file and publishes disagreeing ranges of text flagged with these codes:
==== all three files differ
====1 file1 is different
====2 file2 is different
====3 file3 is different