The only flat polygon you will get is by using glutSolidSphere(radius,2,2)
which means 2 stacks and 2 slices(this is a bare minimum condition).
with this call you will get a rhombus.
however if you want something close to a triangle you might want to try with 3 stacks and 2 slices or vice versa, in which case the result will be a tetrahedron.
I’m writing a new tutorial for beginners to OpenGL programming do visit it