Looking at the plans for CatII insides they do show a 3rd bed. Most modern ships have a pull down bunk bed of std single size. These retract in to the ceiling during the day & are lowered early evening by your steward. Be aware that some older NCL ships have a 'Pullman' mattress on castors that lives under the bed during the day!! Really! Why not ring NCL 0207 591 8016 for peace of mind.
They do NOT show a 3rd bed (sorry typo).
We cruised with NCL last year (Norwegian Jewel) in an inside cabin for 3 people (my son being the 3rd person at 14 years of age). The cabin was compact but well appointed. The third bed was a pull-down bed over one of the other beds (forming something like bunk beds). Cabin stewards pulled-down the bed in preparation for the evening and lifted it in the morning when making-up the cabin. My son is 6' and the bed was fine. Happy cruising.