Solid modeling refers to three-dimensional models. When referring to n-dimensional solids, that means that there are shapes, but it could different dimensions. It could be two or three dimensions. There are ways to apply the Pythagorean theorem to three-dimensional shapes if the shape has a right angle in its triangle.
Just like a 2-dimensional shape, the student would need to know the two sides that have the right angle in between them. Then they could calculate the hypotenuse using the theorem of A squared plus B squared is equal to C squared. For a cube, a line can be drawn to make the right triangle. Then as long is one side is known, the theorem can be applied and the cube can be calculated.