diff3 command compares three files and outputs descriptions
of their differences. Its arguments are as follows:
diff3 options… mine older yours
The files to compare are mine, older, and yours.
At most one of these three file names may be -,
diff3 to read the standard input for that file.
An exit status of 0 means
diff3 was successful, 1 means some
conflicts were found, and 2 means trouble.