Is it obligatory to return the surplus to the insurance company if the doctor prescribed 12 boxes of medicine to be taken once a week, and then 9 boxes were purchased, followed by 6 boxes instead of 3?

If the insurance is permissible, then you must adhere to its terms, which stipulate that you are not entitled to medicine except what the doctor prescribes. It is not permissible to take more than what was prescribed, and if you do take more, you must return it. However, if the insurance is impermissible, then it is not permissible to enter into it voluntarily. But if it is compulsory, there is no sin, and it is permissible to benefit from it up to the amount paid for the insurance. Consequently, it is permissible to take more medicine than what the doctor prescribed, as long as it does not exceed the value of what you have paid.