Hi Alice! It is true that the resort check-in time is 3:00pm and rooms will not necessarily be available before that time. You can check your luggage and any bags with the bell staff at the resort and then also check-in at the front desk. They will then text you when the room is available and you can return at that time.
It is possible to book a room for the night before but that is a large chunk of change to have to do just to get a room early. If this is what you choose to do then I suggest you make sure you notify them the day before you arrive to state that you will not be there until the early hours of the morning so they are aware that you will still be arriving. It is a tempting idea but many families can't add that to their budget.
There are many "must sees" in each park. The roller coaster attractions are always tops on most lists such as Expedition Everest, Space Mountain, Thunder Mountain, Splash Mountain, and Rock n Roller Coaster. Other super things to hit include Tower of Terror, Test Track, Mission Space, Soarin, Pirates of the Caribbean, Toy Story Mania, and Kilimanjaro Safaris. There are so many other awesome attractions not listed here that you need to experience and you can read about them by clicking
HERE.
Don't forget the water parks, Downtown Disney, and all the great recreation opportunities that are available to resort guests as well.