Eviction for Seller in Possession in California

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I recently bought a house in California through a Realtor who included the form "Seller in Possession Addendum" as part of the contract. It was a 60 day escrow and the addendum stated: "Term: Seller to remain in possession of Property for 30 calendar days After Close Of Escrow. Seller has no right to remain in possession beyond this term and may be responsible for court awarded damages if Seller does remain." It is now day 29 in the seller rent back and the seller has not moved a speck of stuff. I was informed by my legal assistant that I had to serve the seller with a 30 day notice to vacate. I thought that I could just serve the seller with a 3 day notice since we did not sign a month to month agreement. It was only an agreement for the seller to stay for 30 days after close of escrow. Is my legal assistant correct? I am not a good outside of the box thinker, but I even offered to move the seller's belongings using my own labor and didn't get a definite response. Please help. I am ready to remodel and turn it into a rental. I am at risk of losing at least a month's rent and at least 1 mortgage payment.