Welcome aboard, Carolina! What a wonderful birthday gift you are giving your son! The gift of a Disney cruise is one that will help you make magical memories that will last forever! I'm excited for your family!
Disney Cruise Line offers some wonderful
Youth Clubs. Three year olds on the Disney ships are able to go to either the nursery (It's A Small World on the Dream and Fantasy, or Flounder's Reef on the Magic and Wonder), or to the Oceaneer's Club and Lab. The Oceaneer's Club and Lab are for ages three to 12, and are mostly for secured playtime without the parents. Counselors are on hand to guide the children in all sorts of fun activities. However, in the Club and Lab, children must be fully potty trained. Counselors are unable to assist with bathroom help.
If it comes time to set sail and you're still unsure of how he will do on his own, go to one of the Open House times for the Club. During Open House hours, parents stay with their children to play. It would be a great time to try it out, and even send him into the bathroom on his own, standing outside the door to see how he does by himself!
The other good news is that if you want to be sure he does have assistance with the bathroom and can still enjoy some time on his own while you sneak off for some adults-only time, the nursery is a great option. There is an additional charge of $6.00 per hour for the child's time there, and it must be reserved in advance. I would suggest booking this ahead of time, as hours do fill up there.
I hope both of those options give you some ideas of what you can expect on the ship. I bet that by May your son might really surprise you with his progress! Please let me know if I can help you with any other questions as you continue planning your exciting Disney cruise, Carolina!