...I am guessing that if it is registered as a M/C--which it better be for smog reasons!--then all M/C rules probably apply, including DOT helmet class and quite probably a full-on class C M/C license.
To find documentation that trikes can be driven in California with nothing but a standard Class C license, you'll find multiple (unanimous) sources online – google
"California trike driver's license" or
"California trike 'Class C' license".
Per
eHow.com, California trike operators and passengers are exempted from wearing helmets: the California Vehicle Code Section 27803 states that drivers and passengers riding in trikes that are fully enclosed, not less than seven feet long and four feet wide and weigh 900 pounds or more are exempt from the wearing of helmets.
