a + b = (-1+0, 2+(-5), 0+(-2)) = (-1, -3, -2)
b + c = (0+21, -5+(-3), -2+0) = (21, -8, -2)
a + c = (-1+21, 2+(-3), 0+0) = (20, -1, 0)
a + b = (-1+0, 2+(-5), 0+(-2)) = (-1, -3, -2)
b + c = (0+21, -5+(-3), -2+0) = (21, -8, -2)
a + c = (-1+21, 2+(-3), 0+0) = (20, -1, 0)