As mentioned above, you don't need to wait until 3 p.m. to check in. Check in whenever you arrive so that your MagicBands are activated for use. And there is always a chance your room may be ready.
If you are utilizing Disney's Magical Express (and put the tags on your checked luggage) your bags will be transported to CBR for you and will be placed in your room when it is ready. You won't need to be there.
You can stow your carry-ons with Bell Services if your room is not ready when you arrive. You can also stow your checked luggage if you opt not to use DME. However, Bell Services WILL NOT put your items in your room unless you are present. (You are also free to pick up your belongings from Bell Services and haul them to your room yourself.)