What vitamins or food supplements do you recommend I take over a period of 3 weeks so that I can be at my best physically and intellectually?

Your studies are demanding, especially at the end of the semester. Please take care to eat well: Make sure your plate is colorful, so lots of fruits and vegetables, which will contain the vitamins and minerals required for proper functioning. I also suggest you eat ideally 2 fish meals per week, this food contains a good amount of omega-3 fats, which are also beneficial. Finally, don't skimp on rest, make sure you get at least 7 hours of sleep a day to rest your brain and improve your ability to assimilate the material you are studying. There is no miracle drug that will meet your needs in the immediate future, just a healthy diet, enough rest and stress management.