Reflector 3 uses a new style license key. This key is much shorter than the original key for Reflector. These are separate license keys that are purchased independently.
If your license key doesn't fit in the registration box, it's likely you've got a Reflector key, not a Reflector 3 key.
New style keys follow this format:
Old style keys are a very long strings of random characters. These keys often exceed 100 characters.
If your registration window looks like this, you're using Reflector 1. Either a Reflector 1 license will be required, or you can download Reflector 3 from here.
Please note Reflector 1 will not work with iOS9 and above.
Reflector 3 registration window:
Reflector 1 registration window: