<p>Look at your question from another perspective. Say the company that offered you the internship contacted you 3 weeks later and said the offer was being withdrawn because they found someone they liked better. How would you like that? And consider the fact that the person(s) in that company that selected you now have egg on their face(s). Also, not only you but anybody applying from your school in the future will be viewed with a jaundiced eye. And if your school had anything to do with arranging the internship, you would be in deep s**t with them too, and deservedly so.</p>