You won't need a hairdryer, soap, or shampoo unless you're picky.
If you don't have a good "wheelie" carry-on bag, go purchase one. Check with the sales person and get the biggest one you are allowed for carry-on.
3 nights? I would bring whatever you think you'll need in clothes but items like shorts and skirts you can wear more than once. That way you don't overpack your little bag. Take extra socks and undergarments as they will get wet, sweaty, dirty, and so on...
If you are staying at the Villas of Wilderness Lodge, you might have access to a washer/dryer. Check with your friend and find out what type of room you have. The VWL are a DVC property but you are probably staying in a studio and I don't think the studio's have washer/dryers.
If you plan on relaxing by the pool you may want to bring a magazine or book to read.
Check with the Airline, you may be able to take a backpack along with your carry on. On United you are allowed a personal item and a carry on. This is supposed to mean a purse or briefcase but I see people bringing small back packs on all the time.
If you told us what your schedule was going to be and what you planned on doing we could probably help you more.
