A deluxe balcony cabin can comfortably accommodate up to 4 people. I have travelled with 2 children for many years sharing one cabin including Oriana. Now aged 20 and 15, they still share with us!
The 3rd adult will be most likely be on a sofa bed which will serve as seating during the day.
As a frequent world cruiser on Oriana I would say that wardrobe space for 3 adults will be limited and stateroom space will be quite cosy.