Well, the beginning of a story is often called the 'exposition', the middle is the 'rising action and climax', and the end is the 'resolution'. These parts work together to form a complete narrative.
The terms for the beginning, middle, and end of stories are respectively the 'introduction', the 'development', and the 'conclusion'. They help shape the story's flow and progression.
The beginning, middle, and end of stories are typically referred to as the 'plot structure'.