Birth control pill help?

So i took the 21 birth control pill (those without the iron) and i usually take those with the iron..i was told when its done i have to wait 7 days for my period to come...the confusin part is am on day 3 and my period has not come yet...when it comes do i start back the seven days from the day it came then start takin the pill or do i wait for when my period ends to start the pill again?