Unfortunately, unlike other hotels, Disney does not have a gauranteed check-in time. They state that check-in begins at 3 p.m., but this does not mean a room will be ready for you at that time. I have checked-in many times when a room is not ready until 4:30 or so. But, I have also checked-in when a room was ready at 10:00 a.m. It depends on occupancy. If the hotel is full, you may not have a room until after 3 p.m.
Good Luck