Try http://www.beyondships.com/NCLSpirit-Tour-6.html. If you look at the Category I inside Cabin, that will be about the size of your outside cabin; you can even see the third birth you will pull down from the wall for your third person. The good news is you will have a window! Also try Webshots, searching for Norwegian Spirit, you'll get some good cabin shots.
It will definitely be tight with the three of you, (espcecially the bathroom situation!), but good friends can always make it work. Have fun!