Will be in Madrid this Fri-Sun (May 30-June1) & wondering if purchasing a 3 day Madrid card is a good idea (3 travelers, 1 of which is a student, age 20). It's not high season yet, but it is the weekend & not sure what lines will be like at Palace & Prado. Also, if I purchase the Madrid card online, is there a way to pre-book an Essential Madrid tour, or do I have to wait until I exchange the voucher for a card when we arrive. Thank you.
I have not tried using www.madridcard.com, but I think you should try it. If it is going to offer you any reservation options, it will do so before you enter payment details. I won't estimate the crowds, but this may now be high season for Madrid, before it gets too hot and people head for the beach.
We were in Madrid for a week last November, and the situation might be different in May/June, but for what it's worth, we stayed at an apartment near the Royal Palace and passed it every day on the way to/from our Metro station. The lines for the palace were always longer in the afternoon, so the earlier you get there, the shorter your lines might be. The Royal Pharmacy at the palace was mildly interesting, mostly thousands of empty glass bottles on shelves, but it was interesting to see the pictures of all the royal physicians going back hundreds of years - changing fashions and hairstyles! If you're rushed for time, see the palace, but the royal pharmacy might not merit your time. Have a great trip!