How effective is the pill after only taking it for 3 days while in your period and having unprocted sex?

i think it depends on when you first started it. i have heard that if you start it on the first day of your period then you are automatically covered and that any other time during your period you are not covered for 7 days. but i have also heard that if you start at anytime during your period your automatically covered so im not sure but you should still use a condom for the first 7 days to make sure