First (nearly) minimal script for execution in FreeCAD Python console

tetrahedron.py

# store as eg. '/home/pi/tetrahedron.py', then execute in FreeCad Python console by:
# exec(open('/home/pi/tetrahedron.py').read())

import FreeCAD,FreeCADGui
import Part
import math

if FreeCAD.ActiveDocument == None:
    FreeCAD.newDocument() 
                      
obj=FreeCAD.ActiveDocument.addObject("Part::FeaturePython","tetrahedron")


r = 5

v=[ FreeCAD.Vector( math.sqrt(8/9),               0, -1/3).multiply(r),
    FreeCAD.Vector(-math.sqrt(2/9),  math.sqrt(2/3), -1/3).multiply(r),
    FreeCAD.Vector(-math.sqrt(2/9), -math.sqrt(2/3), -1/3).multiply(r),
    FreeCAD.Vector(              0,               0,    1).multiply(r) ]

faces=[]

faces.append(Part.Face(Part.makePolygon([v[0],v[1],v[2],v[0]])))
faces.append(Part.Face(Part.makePolygon([v[0],v[1],v[3],v[0]])))
faces.append(Part.Face(Part.makePolygon([v[0],v[2],v[3],v[0]])))
faces.append(Part.Face(Part.makePolygon([v[1],v[2],v[3],v[1]])))

shell = Part.makeShell(faces)

solid = Part.makeSolid(shell)


obj.Shape = solid

obj.ViewObject.Proxy=0
FreeCAD.ActiveDocument.recompute()
FreeCADGui.SendMsgToActiveView("ViewFit")                

Coordinates taken from Wikipedia:
https://en.wikipedia.org/wiki/Tetrahedron#Coordinates_for_a_regular_tetrahedron