Use the program diff3 to compare three files and show any
differences among them. (diff3 can also merge files; see
Merging From a Common Ancestor).
The “normal” diff3 output format shows each hunk of
differences without surrounding context. Hunks are labeled depending
on whether they are two-way or three-way, and lines are annotated by
their location in the input files.
See Invoking diff3, for more information on how to run diff3.