Unlike the US where most hotel rooms will sleep two comfortably and are priced the same whether one or two persons occupy the room, the rates in Europe will almost always be different. Once again, be sure you're clear on the rate the hotel will charge you. You won't be surprised if we tell you a triple is a room for three. Number and arrangement of beds unspecified, though you might expect three twins or a double and a twin. If you want to sleep four in a room you'd be looking for a quad .
Many hotels have none.
Hotels may have cots, and rooms large enough for an added bed. Arrangements differ, but the basic idea is to provide separation between sleeping and sitting areas. It is usually not large enough to sleep four. If it will sleep three the third bed will most likely be a pull-out sofabed. The bedroom and sitting areas may be separated by wall and door, by curtain, or nothing at all.