Hi Sanam,
There are many books available that explain the solving methods for the 3D Schrödinger equation. Some popular ones include "Quantum Mechanics: Concepts and Applications" by Nouredine Zettili, "Introduction to Quantum Mechanics" by David J. Griffiths, and "Quantum Mechanics" by Albert Messiah. These books cover the basics of solving the Schrödinger equation and also provide examples and exercises for practice. Additionally, there are many online resources and video lectures available that can also help in understanding the solving methods for the Schrödinger equation. I recommend doing some research and finding the resource that best suits your needs and learning style. Best of luck with your studies!