In California, is it legal for a company to write-up or fire employees if they have more than 3 sick days (without dr's note but also non-consecutively) in 90 days?

In most cases they can fire you for anything they want. It is called at-will
You can quit at any time for any reason or no reason at all, and an employer can terminate your employment at any time for almost any reason or for no reason.

Employer: At Will Employment FAQ (http://labor-employment-law.lawyers.com/employment-contracts/Employer-At-Will-Employment-FAQ.html#one)

Maraming salamat:)

Maraming salamat? What does that mean?