The 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 -, which tells 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.