Thank you! The three boxes are to indicate you can take the Magical Training stunt up to three times, if you want to choose additional schools of magic.
Oh! I think I get it. The Magical Training stunt only gives a +1 bonus. When I read the description, I thought the +1 was only one additional school. Just to compliment again, I really love the elegance and economy of this game. The design, the flavor...it's all perfect. I cannot wait to bring it to the table.