Is the extra person always someone related to class 3-3? I havent dig too deep, but I think the extra person in all the years when the calamity happens is someone already related to the class 3-3.

Is it true?
Yes, as far as I remember, this is confirmed at the beginning of the manga.

Class 3-3 has been the 'cursed' class since 1972




   Every year since class 3-3 has had an 'Extra' person in the class from the original year, who doesn't realise they are already dead.


The Another Wikia is a good source of information on the series.