Yes, potty-trained 3 year olds can be with the 3 and 4 year old group in the Oceaneers Club or can be booked for a fee in the Nursery.
We sailed with my daughter over her 3rd birthday. She wasn't fully trained yet, so my husband and I put her in the nursery when we went to Palo. She was not happy when we dropped her off and we were sure we'd be getting a call to pick her up, but she settled down quickly. When we picked her up, she was happy to have spent time talking and playing with the Nursery cast members, and they seemed to be happy to be entertaining an older child who could carry on full conversations.