Plan B is only effective if it's taken within the first 72 hours of having sexual intercourse, and even within that first 72 hours, if conception occurs before you take it, then it's also no longer effective. So if it's been more than 3 days then it should not be taken.
Plan B does not work after 3 days. This is because the manufacturer recommends that it is taken within 72 hours of having unprotected sex. If conception occurs, then this contraception pill becomes no longer effective. You should visit your doctor for more help.