Let $f(x) = x^3 - 3x + b$ and $g(x) = x^2 + bx - 3$, where $b$ is a real number. What is the sum of all possible values of $b$ for which the equations $f(x) = 0$ and $g(x) = 0$ have a common root?

Let's think step by step and output the final answer within \boxed{}.
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