A contradiction
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No because 2d shapes are plane such as polygons but example of 3d shapes are: pyramid, cone, cuboid, cylinder, sphere ... etc
A right angle triangle is a plane figure not a 3d one.
Use the coordinates of the vertices.
The cross section, of a 3D object, is the 2D shape made when a 2D plane cuts across the 3D object. Often "cross section" refers to the shape made when the plane is at right angles to an axis of the 3D object but this need not be the case.
An irregular polyhedron if it has plane faces. If not, then there usually will not be a name for it.