Critical points is where the derivative (slope) is zero or does not exist. So to do this we have to find the derivative of our function:

So we apply chain rule:
=

Set our first derivative to zero and solve for x:
3(x^2 - 1) * 2x = 0
So we can see that (by plugging in) 0, -1 and 1 makes our solution true
So our critical value is x = 0, x = -1, x = 1