How old are the 3 extra's in the cabin, are they small children or teens and older?
The bed lay out will probably be 2 upper beds which fold into the wall during the day, 2 lower beds and then a roll a way cot brought in. It will definetly be a tight squeeze, but I have seen it done. The cabins are approximately 165 sq. feet. The biggest obstacle you will have is when its time to shower and dress..not much room in bathroom and not enough room in cabin for 5 people to move around in, but you can manage if you do it in shifts. Since you are on the Fantasy you are going on either a 3 or 4 night cruise, so you should be able to manage, I would defintely not recommend it if you were going to be on a longer cruise.
You should do fine, just plan on having a great cruise. Is this your first?