Well Ed.. I held off on answering hoping that someone with experience with these exact categorys/ship would answer you. but since that did not happen I will answer with "general cruise knowledge"
We travel with 4 to a cabin (two young kiddos) and therefore book cabins that accomidate 4 people. If your BD cabin states it can sleep 3 people, the sofa will be convertible to a bed. OR there should be a pull down berth for your daughter. OR, some cruise lines will bring in a roll-a-way bed for the extra people. My guess is that in that category, you will have a convertable sofa.
When you go to book that cabin, if the room can not accomidate the extra person, the agent will not be able to assign 3 of you to the cabin. And the agent should also be able to tell you from the room description what the 3rd/4th person sleeping means is...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013