Which of the following shapes has three sides?

The shape that has three sides is the triangle. A triangle is defined as a polygon with three edges and three vertices. It is one of the simplest forms of a polygon and can vary in terms of the lengths of its sides and the angles between them. This basic geometric structure serves as the foundational shape for many constructions and designs in both art and architecture.

In contrast, the other shapes mentioned do not fit this criterion. A square has four equal sides, a rectangle also has four sides but with opposite sides being equal, and a pentagon consists of five sides. Understanding the distinct characteristics of different geometric shapes allows for a clearer grasp of basic geometry concepts.