A three-sided pyramid is called a triangular pyramid or a tetrahedron. In geometry, a tetrahedron is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. It is the simplest of all the ordinary convex polyhedra and the only one that has fewer than 5 faces.
The term tetrahedron comes from the Greek words ‘tetra’ meaning ‘four’ and ‘hedra’ meaning ‘face’. This name reflects its structure, as it has four triangular faces. A triangular pyramid, on the other hand, emphasizes the base shape, which is a triangle, and the fact that it rises to a single apex point.
In summary, whether you call it a triangular pyramid or a tetrahedron, you’re referring to the same three-dimensional shape with three triangular sides meeting at a common point.