Let x, y, z be positive reals. Which of the following implies x = y = z?
(I) x3 + y3 + z3 = 3xyz
(II) x3 + y2 z + yz2 = 3xyz
(III) x3 + y2 z + z2 x = 3xyz
(IV) (x + y + z)3 = 27xyz