f(x-1) + f(x) + f(x+1) = \left[(x-1)^3 - (x-1)\right] + \left[x^3 - x\right] + \left[(x+1)^3 - (x+1)\right] = [x^3 - 3x^2 + 3x - 1 - x + 1] + [x^3-x] + [x^3+3x^2+3x+1-x-1] = x^3 - 3x^2 + 3x-1-x+1+x^3-x+x^3+3x^2+3x+1-x-1 = 3x^3 + 3x