How can I access Prisoner of Azkaban on Pottermore?

당신 don't unlock it. It's just like the real thing: 당신 would have to wait just like 당신 wait for the 다음 book 또는 movie.

당신 can't. It's not out yet :(

U can't unlock it yet... u'll have to wait :)
당신 can check on the 포터모어 blog... they update what's happening on 포터모어 and might have the 날짜 of when the 3rd book is coming on pottermore... not sure, but worth a shot ;)