My friends and I will be spending 4 days in each place. What are the absolute must sees and places to eat? Thank you in advance for any advice!
Sights depend on your personal preferences. Madrid has three important art museums, but if you don't like art... many people would say a lot of Barcelona's highlights are related to its modernist architecture. How do you feel about funky buildings?
Sevilla is the best place for tapas, but there are great places in Madrid and Barcelona too. Any guide book or website will give you the sights, many more than you can possibly fit in. Choices need to be made by personal preference. So my advice is to buy a guide book and go over it together with your friends.