Two: (conceptually) a line segment is two points on a straight line and the part between them; A sector is a figure surrounded by an arc and two radii passing through both ends of the arc; A cylinder is a figure obtained by rotating a rectangle around one side.
Three: (in nature) the line segment is finite, and it has two endpoints; The distance from the vertex of the sector to any point on the arc is equal; Cylinder A cylinder consists of two bottom surfaces and one side surface.