Register vehicle before it's at dealer, no chance. Anything could happen while motor is in transit, optional extras might not be fitted as per your spec, car might be or get damaged in transit and as I had many years ago with new car order (not Audi) when it arrived it was a 5 door, I ordered a 3 door, salesman was so embarrassed but not his fault, complete balls up during production.
You need to see your S3 as soon as it arrives, make sure it's in mint condition, so just wait a little while longer, dealer gets it registered within 24hrs and before you know it you will be driving it.
It's certainly worth the wait, don't rush it, enjoy.
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