Constructing regular expressions with given substring requirments
Hint: Replace each sub-expression of the form $\Sigma^* A \Sigma^* B \Sigma^* C \Sigma^*$ with a union of expressions of the form $\Sigma^\alpha \Sigma^* A \Sigma^\beta \Sigma^* B \Sigma^\gamma \Sigma^* C \Sigma^d \Sigma^*$ for appropriate $\alpha,\beta,\gamma,\delta$.

Notice also the point of the exercise, namely that (plain) regular expressions aren't a good fit for describing passwords. We would really appreciate being able to intersect regular expressions (do you see how that helps?).