A polyhedron is a solid shape comprised of flat surfaces, such as a cube.  A regular, convex polyhedron is a solid shape where the surfaces or faces, edges and angles are all equal in length.  It is convex because the faces are flat in that they do not dip or dent inward.

A three-sided pyramid is called a tetrahedron.  It is one of the Platonic Solids.  A Platonic Solid is a three-dimensional shape comprised of sides or faces that are all the same shape and size and the same number of faces come together and meet at each point or vortex of the shape.  For instance, a tetrahedron (three-sided pyramid) has three triangular faces that are all the same size and at each point, three faces always meet together.  

Plato wrote about these three-dimensional shapes in Timaeus, 360 BC.  He associated each shape with an element.  The tetrahedron was associated with fire.  The cube (four faces comprised of squares) was associated with Earth.  The octohedron (eight faces) was associated with air.  The icosahedron (20 equilateral triangular faces) was associated with water.  

The Platonic Solids are the basic geometric shapes that comprise our physical universe, such as cells, flowers, honeycomb, crystals, etc.

A Merkaba is comprised of two tetrahedrons.  As you can see in the famous drawing called The Vitruvian Man by Leonardo Da Vinci, man lives within an energetic merkaba.  Energy moves up and down through the angles and faces of the merkaba.  This is why a merkaba is considered a tool of ascension.