What should I do about my family's refusal to approve my marriage except under impossible conditions, and my lack of an official divorce certificate from my ex-husband, and is it permissible for me to marry without their knowledge and without the divorce certificate?

If you are certain that you have been divorced three times, then you are irrevocably divorced from him, and it is not permissible for you to remain with him. You must observe the 'iddah (waiting period), which is three menstrual cycles or three months. If your husband refuses to declare the divorce, refer the matter to the judiciary to request Khula' (divorce at the wife's instance) and obtain a divorce certificate. Do not marry anyone else until you obtain this certificate. After the 'iddah has expired and you have the divorce certificate, your family has no right to prevent you from marrying a suitable man. However, you are not permitted to marry yourself off without a guardian (wali). If the guardian refuses to give permission (adhl), another guardian from your family may marry you off, or the judge may do so.