For the classic ships (not sure about the new ones) the bed can be pulled apart to make two twin beds and then someone can be on the couch. Maybe someone with more info can write in.
They should be able to separate the beds.
Only on the Magic and Wonder.
The Dream and Fantasy beds do not separate. Three persons wishing separate beds would have to use the pull-down or murphy bed AND the sofa bed. (I don't know if the pull-downs are big enough for adults but I know not all staterooms have them. If you book a stateroom on the Dream or Fantasy that sleeps 3, be aware that they probably have only 2 beds).