K_3 is a complete graph in graph theory that consists of three vertices, with each vertex connected to every other vertex. This means there are three edges in total, forming a triangle shape. K_3 is the simplest example of a complete graph and serves as a fundamental building block in various areas of mathematics and computer science. In terms of properties, K_3 is a planar graph, meaning it can be drawn on a flat surface without any edges crossing. It also has a chromatic number of 3, indicating that three colors are needed to color the vertices so that no two adjacent vertices share the same color.