Square pyramid
A tetrahedron (triangular pyramid).
pyramid
A rectangular based pyramid would fit the given description
duodecahedron is a 3D shape that has 20 triangles as it's base
A tetrahedron has four equilateral triangles as sides
Square pyramid
A Triangular Pyramid has 4 triangles and 1 square.
The simplest such shape is PYRAMID.
Tetrahedron or Triangular Pyramid
There can be no closed 3D shape with only one square and 3 triangles.
A tetrahedron (triangular pyramid).
square based pyramid
There is no 3D shape with 6 faces: 4 rectangles and 2 triangles. See the link for all forms of hexahedra.
A 3D shape with all triangular faces can be a tetrahedron pyramid.
A three-sided pyramid, also termed a triangular prism.
pyramid