Is it worth saying whether diff3 --help and diff3 --version output to stdout or stderr?
GNU diff3 has -v as a --version alias. Does the replacement want/have that? If so, should be added to the manual page.
You should be able to use .Nm only after the first time. This is in fact recommended. https://docs.freebsd.org/en/books/fdp-primer/manual-pages/#manual-pages-sample-structures.