I'm a nurse in CA working the 3pm - 11pm shift. Our parent co says it does not pay shift differential for working the evening shift. I thought it was mandatory. Am I wrong or are they?

Sorry, you're wrong. There is NO state which requires a shift differential for working undesirable shifts. Unless there is a union contract requiring it; in which case the failure to pay would be a breach of the union contract, not a violation of wage and hour law.