How to use one script for diff and diff3 in Subversion?

Question

I have a perl script that determines what external diff to use depending on the file's content. I would like to use it determine external diff3 also.

Which is the best method to know how my script is being called?

I was planning to catch the last three parameters, and verify if they are all files, to use the diff3 options, but I'm hoping there is a cleaner method.