For health insurance purposes, "dependent" means "anyone who is eligible to be covered on the insurance and is not the employee/subscriber him/herself. It's got nothing to do with tax dependency.
Once more. With feeling. There is NO California law that entitles an ex-spouse to continue to be covered on the employed spouse's insurance. There is NO California law that entitles an ex-spouse to be enrolled on the employed spouse's insurance as a dependent, tax or otherwise, after the divorce is final.
There is NO Federal law that entitles an ex-spouse to continue to be covered on the employed spouse's insurance other than through COBRA. There is NO Federal law that entitles an ex-spouse to be enrolled on the employed spouse's insurance as a dependent, tax or otherwise, after the divorce is final.
There is NO law in any of the other 49 states or the District of Columbia that entitles an ex-spouse to continue to be covered on the employed spouse's insurance. There is NO law in any of the other 49 states or the District of Columbia that entitles an ex-spouse to be enrolled on the employed spouse's insurance as a dependent, tax or otherwise, after the divorce is final.
This is true regardless of the employer or the health carrier. There no is NO legal provision guaranteeing an ex-spouse coverage on the employed spouse's health plan no matter what size the employer is, what state they are in, what industry the employer is, or who the health insurance carrier is.
An employer may, if they choose to, permit such coverage as long as it is clearly spelled out in their coverage plan documents. I have yet to hear of any employer who had chosen to do so with the sole exception already listed above.
The reference to three years leads me to believe that COBRA was elected at the time of the divorce and is now up. If you are the employed spouse and you are required to provide coverage to your ex, you're going to have to find it other than through your employer. If you are the ex-spouse, what happens now depends on what was in the divorce order, but a court CANNOT order the employer to cover you outside of its own rules. And if you are the employer, you should already know what your plan document provides for but unless it expressly states that ex-spouses are eligible dependents, you not only need not but must not cover them.
I do this for a living. I've heard all the questions there are. If the answer is still not clear, then explain in detail what the situation is and I will answer it. But if you are looking for a legal loophole that will force the employer to provide coverage to the ex-spouse, there isn't one.