How effective is plan b if took on day 3?

Plan B: is most effective when taken within the first 24 hours of intercourse and loses some effectiveness the further out it is taken. It is still effective 72 hours from intercourse.

Up to 89%: Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes.