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Generally, the WA corp. is doing business in California, so the WA corp. will have to have California withholdings for wages paid to people who work in California.
You don't have to worry about out-of-state employees, but they fix things when they file their WA or OR or CA tax return. The CA residents have state tax withheld, no issues there. The WA and OR residents have to file CA non-resident tax returns, and their state tax returns (where they can claim a credit for the CA taxes paid to CA as non-residents).