Hello, I'm a graduate student in this year's exam. We are from the Department of Mathematics. Relatively speaking, Math III is relatively simple. But that doesn't mean it has nothing to do with high school. For example, the formula of trigonometric function, the ability of spatial imagination and the exercise of logical thinking are all reflected in the postgraduate entrance examination.
Moreover, the part of probability and statistics is only reflected in high school knowledge, which lays the foundation for further study in universities in the future.
However, if you really don't have time, it is ok to study college mathematics directly. As long as you study hard and practice more, you are sure to get good grades. I wish you early success.