"Hagrid must have forgotten to tell him something, like tapping the third brick on the left to get into Diagon alley".
Where exactly do you find this third brick on the left? Do you count bricks from the place where you are standing or from the left end of the wall?
UPD: Почему мне самой не пришло это в голову, я не знаю. Однако вопроса об адекватном переводе предложения на русский это не снимает.
Which brick it is isn't important. Harry isn't actually thinking of trying the third brick, even if he knew which course it was on. The idea is that it's the typical treasure hunter's clue - twist the third bunch of grapes on the carving round the fireplace; stand in front of the doors and say "Friend"; knock three times. So if the language you are translating into has the same sort of literary convention, the iconic 'third brick on the left' will do without being specific.