For each science you take 3 exams
Biology: B1, B2, B3
Chemistry: C1, C2, C3
Physics: P1, P2, P3
These aren't topics. Each exam covers many topics. There isn't any specific focus. People who only do single science on do the "1" exams (B1,C1,P1), those who do science and additional science do the "1" and "2" exams. Some say that the 3rd exam covers harder topics, but I don't think so.
The first exam is just core knowledge, the second unit is building on the core knowledge, and the third unit is just more stuff.