Imagine the following three files. B is the common ancestor of both A and C.
test foo test
test test test
test bar test
Now call it:
merge a b c
You will get an error:
merge: warning: conflicts during merge
And A will now contain conflict markers:
test <<<<<<< a foo ======= bar >>>>>>> c test
If you use diff3, the output is slightly different, and it is printed to stdout instead of written to a file:
diff3 -m a b c
test <<<<<<< a foo ||||||| b test ======= bar >>>>>>> c test
Given an appropriate text formatting rule, this may make perfect sense. It is certainly a more HumaneInterface than being presented two edit boxes. For non-trivial pages that is far too difficult to resolve manually.