Ahoy there Lucy and thank you for bringing such a great question to the Disney Parks Moms Panel.
You most certainly are a very lucky duck indeed with your exceptionally magical 3 week long Walt Disney World Resort ~ Disney Cruise Line ~ Walt Disney World Resort holiday, it's sure to be practically perfect in every way!
I'm happy to share with you that in my own experience during multi Walt Disney World Resort hotel stays Bell Services can for Guest convenience transfer luggage between hotels on your behalf.
However, because I have yet to indulge upon a magical sailing with the Disney Cruise Line I wanted to seek the advice of
Disney Parks Moms Panelist Diane; a Disney Cruise Line Specialist who advises that in her experience Guests may indeed store a reasonable amount of luggage with Bell Services whilst they cruise. It's worth mentioning that Diane also told me it may be wise to first clarify your families particular storage needs with Bell Services to avoid any unnecessary confusion.
If I can help sprinkle any extra pixie dust onto your already magical holiday plans to visit Walt Disney World Resort and sail with the Disney Cruise Line please pop back to ask me or perhaps a
Disney Parks Moms Panel ~ Disney Cruise Line specialist another question.
Have a great Disney day.
~ Jane