I deduce that you've only worked with a limited type of programmer. I have, indeed, tested code written by people who did not do as you suggest. That being th

I deduce that you've only worked with a limited type of programmer. I have, indeed, tested code written by people who did not do as you suggest. That being the case, I wrote code to produce all the test cases, including all the permutations of the elements. In this case I would produce three numbers in non-decreasing order, and then output all six permutations of them. I write safety-critical software, and I test safety-critical software written by others. I fully expect all of these cases to be tested, including feeding the program with non-numeric values, negative values, junk, and floating point numbers that are equal under various levels of precision. If you're not doing this, you're deliberately choosing to sacrifice the program's quality for your time and effort spent testing. That's a choice you can, or course, make, but you should be very clear to yourself, if not your users, that you're doing it.