A hexahedron is the name for a 6 sided 3D shape.
A nonagon is a 2D shape and there is not a 3D equivalent shape.
the name of a 20 sided 3d shape is called a icosahedron
A cube is a 3D shape with a top and bottom.
There are many possible shapes. For example, a heptagonal pyramid, a hexagonal prism, a rectangular dipyramid.
The simplest such shape is PYRAMID.
It is important to learn and remember the name of 3D shapes. A 19 faced 3d shape is called a nonadecahedron.