All of the questions you ask can only be answered by the B&B/hotel itself. How old is the third person? When my son was very young and small, he would sleep in the bed with us especially since most B&Bs have one large bed for two people (two singles put together). Whether or not we could do that and if we had to pay extra was dependent upon the individual B&B. The only establishments that generally have a roll away bed was a hotel. We did have that set up at a Hilton. However, again, that just depends on the hotel. Some hotels have very strict fire code and will not allow three people in the room. It is not like here in the States where you can get a two queen/full room and put four people in the room.
On my most recent trip with a travel partner, we stayed at B&Bs that were a triple or a family room because we requested two separate twin beds. I would go on the B&Bs website and search their photos looking for such a set up. Also, some of those set ups end up being a small apartment. In Germany and Austria a small apartment is called a Ferienwohnungen. You can usually find these in popular tourist destinations such as the Black Forest, ski resorts and on the outskirts of towns/cities.
Again, the only way to find out if a third person can be accommodated is to email the establishment.
As far as a roll away bed being comfortable. Are they comfortable here in the States? They don't seem to be any different in Europe.
Hope this was helpful. Have a great trip.