(default keyboard shortcut Shift T)
The Triple command (Multiply > Subdivide > Triple) converts the selected polygon(s) into triangular polygons. For example, a square polygon becomes two triangular polygons. Triangles cannot by definition become non-planar; therefore, this tool is useful for objects you wish to animate using bones, or objects that will have Displacement Maps. Tripling non-planar polygons makes them planar.
Tripling does not change the general shape of the object/polygons. Also, once a polygon is a triangle, tripling it again has no effect.