The sex and prevention of pregnancy question?

so I know that when you hae unprotected sex and you haven't been taking the pill regularly the more likely you are to get pregnant... but how likely is it? like if you miss three days in a row does it go down to 100% from a .99% when fully on it? Also, if you have sex and take like 3 birth control pills the next day... does it still work like plan b?