Numbers one, two and three are required by different majors. Generally, science and engineering students take the numbers one and two. For example, the number three is also called economic mathematics, because economic candidates basically take the number three, and some schools require the number two. In terms of difficulty, three is the easiest. Specifically, you need to check the professional catalogue of the school you want to take the exam.
Why did you hear that there was no unified examination? Because there are some mathematics, such as agronomy mathematics, which are aimed at certain disciplines. This is a non-unified exam, and the topic is given by the school itself.