If a girl were to forget her birth control for 3 days in a row, how likely is it for her to get pregnant?

When is a girl most fertile? I have heard that it is right after her period, but I don't know. My friend forgot her pill for 3 days after she ended her period. She had all three days.. She went to the doctor and is supposed to take the preg test tomorrow morning but just out of curiousity how likely is it that she could be pregnant?