There is no 3-week internship. Most internships last 9 – 12 months. Check out the Academy of Nutrition and Dietetics’s dietetic internship list. Or are you looking for a dietitian that your niece can shadow at work to learn what a dietitian does on a typical day?
I have a niece studying clinical nutrition and is looking for a 3-week internship in May. Do you have a program? Do you know who she might contact?