There are some bargains going at the moment and a single occupancy cabin could be offered at a very good rate but you don't say when you are contemplating this cruise. Separate bedrooms don't exist on very many ships if at all without it being a suite so as has been mentioned you are probably going to have to look at two cabins but there is no reason why one shouldn't be a inside cain and the other an outside or balcony, close together so that you can all use the outside one for a large amount of time, that way you can reduce the costs. But really try bargaining as there are three of you. ..... Neil.
I hope you are asking about a cruise ? If so, she will have to pay a single supplement to occupy a cabin on her own. Very few cruise lines have single cabins but try Fred Olsen cruises or a specialist cruise agent. try ringing the freephone number above for some help !