If you begin calc III being sharp with integration and differentiation, then that will serve you well. It would probably help to get a head start on vectors and their properties/operations and how to represent them in 3-space (e.g. left-handed and right-handed coordinate systems). Getting familiar with vector-valued functions will also be useful. Conic sections will also come up a lot, so being familar/comfortable with the equations of conics (e.g. paraboloid, ellipsoid, hyperboloid, etc.) in 3 variables will be useful.

IMO, some good resources are Paul's online notes for calc III (his worked out examples and practice problems really helped me learn) and Calculus by Larson and Edwards (they have a helpful chapter that covers vectors and conics). Also, I found the youtube channel of the user "mathispower4u" to be extremely helpful when studying calc III (in particular, I thought he did an excellent job on covering line integrals).

Good luck with calc III.