Which 3xxx Math course you are required to take varies depending on your university. Typically it is the higher level version of a lower level course you have already taken. The reason for this is because different schools prioritize different math techniques and they believe the lower level course was insufficient teach the techniques at a high enough level.
The requirements for taking the 3xxx Math course is to have taken all of the 2xxx level math courses. Obviously, this includes the 2xxx version of the 3xxx course. Your notes and material from the lower level version of the course should serve as a good guide for the 3xxx version.
There are two main 3xxx Math courses that are required:
Vector Calculus: is the higher level version of Multivariable Calculus. It involves more formal definitions and uses of vectors and aids in vector heavy courses like Electricity and Magnetism I and II.
Differential Equations: is the higher level of Introduction to Differential Equations. It covers boundary value problems, Laplace Transforms and Fourier Transforms at a higher level. These aid in courses such as Optics and Quantum Mechanics I and II.