Spela, I am in France. 3 days is really fast, it's rare for a package to arrive so quickly from Europe to the US. I'd have to pay tons of money to ship my package for a 3 day delivery, my best option here is 5 - 7 days expedited shipping (still expensive but doable) and I tell my customers 5 - 15 days on average, could take longer. Did you tell your buyer it would arrive in 3 days or did you tell them that it could arrive in 3 days but may take longer? Just making sure you did't guarantee the shipping date because if you did, that's a different issue.
If you did not guarantee, then your buyer needs to wait. It could be stuck in customs.