Left Mouse Button
Used to pick or select objects in drawing AutoCAD and perform as following:
Step 1: Click the left mouse button to select an object area in the drawing.
Step 2: Press ESC twice to deselect an object (or to cancel a command).
Right Mouse Button
When you click the right mouse button to repeat last command or end current command. In addition, When you click SHIFT + right mouse button will access objects snap shortcut menus.
Middle Mouse Button
When click double middle mouse button to zoom all objects in drawing. In addition, when click and hold on middle mouse button, you will move the view planar to the screen.