No way they will let extra teen in a double room

Unless it is a US chain hotel.  The european ones will not allow this.  You can ask for extra bed or a 3 person room.  But better make sure they know you booked a room for 3 or you will have problem checking in.

Plus the double rooms are really for 2, the beds and rooms are so small there is no way it will sleep 3. Unless you book a US brand hotel.