It appears they offer a child price of up to 12yrs so looks like they can accommodate a 3rd bed. But, it's unusual to have 3 adults in a cruise cabin.

"Children Reduction Policy :
* Children from 6 to 12 years sharing in parents cabin ( with an extra bed ) are granted 50% discount on the adult rate ( per person in double room )"

Depending when you are travelling - some of the cruise ships and hotels offer no single supplement for cabins and rooms during the main summer months (June - August).

CwB