The Big 3 is a group of three third-year students who are among the best students in U.A. High School.
The Big 3 of U.A. represent Japan's Top Hero Candidates. How they are selected as candidates is unknown as the current group does not have any notable feats or academic ranking to distinguish their abilities. So it can be assumed that progress in academics or school related work are not primary factors in their selection.
However, all three are shown to be very powerful outside of their academic knowledge.
All three are said to have potential exceeding that of most pro heroes. Mirio has the potential to be number one, Nejire can defeat powerful villains singlehandedly, and Tamaki's ability alone outclasses most pro heroes.