You can register in nearly any of the 50 states. It seems dumb, but it doesn't matter which state you use to take Step 3, except for the fact that some state medical boards have constraints on when you are eligible to take the exam (some require you to have COMPLETED your intern year, for example.) But, once registered, you can take the exam at ANY Prometric testing center (even if not in the same state that you registered with and even if the state where you are taking the test is one that requires you to have completed an intern year, but you registered in a state that does not.)
