If my tenant refused the certified demand letter I sent her for the back rent what are my other options if I am going to sue her

The 3-day notice is sufficient. You do not need another letter. You can seek to evict her and recover unpaid rent in the same lawsuit and you do not have to file a separate small claims suit. The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change. Attorney is licensed to practice law only in the State of California. Responses are based solely on California law unless stated otherwise.

You don't need a demand letter at all to start the process, whether sent certified mail or not. Hire a lawyer or get a guide like the one linked below to make sure you don't make any more mistakes and get this procedure right the first time. Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.