You do not have to wait until 3 to check in, you can check in any time after 7 am. Your room might not be ready, you can either keep your bags in your car or store them with bell services and have them sent to your room after you get there. I do split stays a lot and check out of one resort early in the morning and drive to the second resort and check in then hop disney transportation or walk to Epcot if staying at an Epcot resort and go about my day. They will text me when my room is ready and when I'm ready to go back which is not always when they text me but later if I've left my bag in my car I go get it and go to my room or if I've left it with bell services, I go to my room and call them and let them know they can bring the bag up.