Since a frustum is a portion of a solid, three-dimensional figure, and a rectangle is a plane, two-dimensional figure, there can be no such thing as the frustum of a rectangle.
square and triangle
There is not such thing as a three-dimensional triangle. You are thinking of a tetrahedron which is a three-dimensional figure with each side being a triangle. This has four vertices.
I don't think there's a three dimensional rectangle, and if the two dimensional triangle exists in three dimensional space, it would have strange properties because it only has 4 edges. There however, a three dimensional rectangular prism. It has 12 edges.
A triangle is not a polyhedron. A triangle is a two-dimensional figure, and a polyhedron is a three-dimensional solid. An example of a triangular-shaped polyhedron is a pyramid.
A cube is a three dimensional object with all sides of equal length. A rectangle is a two dimensional figure with two pairs of sides equal in length. Three dimensional is like a rubrics cube, something you can hold. A two dimensional figure is something you can not hold, a drawing is two dimensional.
A square pyramid
An isosceles triangle.
It is a figure that is flat. For example: triangle, square, pentagon...ect. A three dimensional figure is like a sculpture. Example: pyramid, cube prism...etc.