What are the procedures under CA child custody laws regarding custody and visitation

Custody and Visitation in California: I have had physical custody of my three kids for 13 yrs the oldest is 15. Last month I filed for a support modification (after waiving $50k in back support) and in retaliation their dad has filed a mod for physical custody and cs against me. While my kids were visiting him he had them write letters that they wanted to live with him. 2 youngest did and oldest didn't.

We go to court in a month. How likely is it that he will get custody of them and do you think it is mandatory I have an attorney file my response. He is self represented.

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