Hi - wow - you do have a lot of planning to do in a little bit of time, but don't sweat it. You've cruised before so you know how things work on the ship - even though you'll be on the Magic, much will be the same. What you can expect to be different on a 7-night vs a 3-night is that you will feel like you have time to actually relax a bit - 3 days is quick! Also be prepared to say you'll never do a 3-night again. LOL.
Formal night in the Med tends to be less formal the Formal night in the Caribbean. It's still nice dresses or pants outfits for the women, jackets and ties for the men. Some people do the whole formal thing, but for the most part, with luggage limitations from the airlines, people are less likely to pack their formal wear for the trip.