Actually, you do need all 3 part of COMLEX for California.
As a DO, your licensing board will be the Osteopathic Board of California.
Article 4. Physician and Surgeon Applications
§ 1611. Credentials.
(a) Diploma. A photocopy of a diploma from an approved school of osteopathic medicine or other proof of having received a diploma shall be furnished by the applicant for permanent filing with the Board.
(b) Transcript of Grades. A certified transcript from osteopathic college shall be requested by the applicant and mailed directly from the school to the Board's office.
(c) Internship Certificate. A photocopy of an internship certificate from an approved hospital shall be furnished by the applicant for permanent filing with the Board if internship was completed prior to July 1990.
(d) Written Examination Results. The certified results of the National Board of Osteopathic Medical Examiners, Inc. (NBOME) or Federation Licensing Examination (FLEX) when applicable, or other State's written examination results shall be submitted to the board and must be sent directly from the certifier.
(e) First Year Postgraduate Training. The first year postgraduate training form (Certificate of Completion of Accreditation Council for Graduate Medical Education (ACGME) Postgraduate Training or American Osteopathic Association (AOA) Rotating Internship OMB.3.1 Rev. 04/00), shall be certified and submitted directly from the Director of Medical Education to the Board.
http://www.ombc.ca.gov/laws.htm
As you can see in part d, you will need to complete the COMLEX series (the test administered by the NBOME). FLEX isn't available anymore since it was replaced by the USMLE. Don't know any states that administer their own written test. Also, you can't be licensed by the Medical Board of California even if you take all 3 steps of USMLE.
The 5 states mentioned above are known for specifically requiring an AOA-approved internship year as a condition for unrestricted licensure.
*PS - starting with the Class of 2008, you will need to pass Step 1 and 2 of COMLEX in order to graduate (school and AOA requirement). Why not just take Step 3 of COMLEX and not worry about that aspect when it comes to licensure? Besides, no one is stopping you from taking all 3 steps of USMLE.