How about averging your three coordinates?:
x = (x1 + x2 + x3) / 3
y = (y1 + y2 + y3) / 3
z = (z1 + z2 + z3) / 3
->
x = ( -3.1 - 19.2 + 14.9 ) / 3
y = ( 21.2 - 8.9 + 15.6 ) / 3
z = ( -1.1 - 12.4 + 39.4 ) / 3
Not sure it will work, though..
x = (x1 + x2 + x3) / 3
y = (y1 + y2 + y3) / 3
z = (z1 + z2 + z3) / 3
->
x = ( -3.1 - 19.2 + 14.9 ) / 3
y = ( 21.2 - 8.9 + 15.6 ) / 3
z = ( -1.1 - 12.4 + 39.4 ) / 3
Not sure it will work, though..