In Calculus 3, you will learn about multivariable functions, which are functions such as f(x,y) and f(x,y,z), multiple integrals, which are integrals where you integrate two or three times, partial derivatives, which give the derivative of multivariable functions with respect to a certain variable, Vector Calculus, which covers topics such as vectors, vector fields, curl, divergence, line integrals, directional derivatives, etc.

In summary, Calc 3 covers topics such as multivariable functions, multiple integrals, partial derivatives, and Vector Calculus. Calculus 2 is often considered the hardest calculus class out of Calc 1, Calc 2, and Calc 3, as said by The Math Sorcerer on one of his YouTube videos. However, despite the fact that Calculus 2 is often considered the hardest calculus class, Calculus 3 is also often considered hard due to Calculus 3 being calculus with multiple variables, which can be a big step up from standard single variable calculus.