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. Once again, if you have an interest here you'll have to get specifics from the hotel. They obviously mean a great deal to the hotel.