I find that when making reservations, especially in Europe, sometimes I have difficulty putting 3 guests in a room. It will just say that they have no rooms that can accomodate 3 guests. So I've just always changed it to 2 and so far I haven't ran into any trouble. But, my daughter is only 7 and she is still small enough to squeeze into a queen or king bed if it's a single room.
So, I was just wondering when I should expect that I should start making sure to count her separately.