Great question, Walter!
Even after three nights of dining, your wait staff will feel like family! On your third evening, just mention to the wait staff that you will be dining elsewhere on your last night aboard the ship. If you would like to give them their
gratuities at that time in order to also extend a personal thank you, it would be a nice way to express your gratitude for the fine service that I know you'll receive!
You might also extend your gratefulness on the last morning aboard the ship during breakfast!
Whenever we plan to dine elsewhere, we try to let our wait staff know ahead of time. Sometimes they'll go ahead and get favorite drinks placed on the table (especially for children!) before your arrival so it's a nice courtesy to let then know that you won't be attending.
You can pay your gratuities via your onboard account. I suggest visiting Guest Services early in your cruise to avoid the long lines that are inevitable on the last night at sea.
Enjoy your cruise! The Disney Cruise Line is full of wonder and magic and dreams come true!