Depending on the institution, there may be on-campus services that can help you with your diagnostic needs. Social anxiety is a pretty straightforward medical diagnosis, but AS/HFA is a considerably more complex issue.
What's at issue really isn't your diagnoses, but rather the accommodation that's required to assist you. I wouldn't fret too much about what's on your form, and consider more what it is that you are going to need from the institution to ensure that you can perform to your best ability academically. Going in with a clear understanding of your needs is far more important that have a label to pin on the place that these needs come from.
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--James