I do not want the answer, I just want what I am doing wrong and hints on how to fix it.
Problem 1: You are using testscore variable before it is being initialized.
Problem 2: You are not performing error handling with atof. I would suggest to use strtod(). You can perform some error handling with it to know that the third argument is a float or not. You can also create your own implementation of atof() which will convert and report error in conversion, if any.
Hint: Try to first check that the number of arguments passed to the c program. After that, try to convert third argument to float using strtod() or your own implementation. If it successfully converts, assign the result of float convrsion to test_score and print last_name and testscore. If not, then consider third argument as last_name and print first_name and last_name.
