What is incumbent upon one who vowed to fast three months if he returned to masturbating and watching pornography, and he has done so multiple times and is unable to fast?

You must sincerely repent from watching forbidden websites and movies and from practicing masturbation. As for your vow to fast for three months if you practiced masturbation, it is a vow made out of obstinacy and anger. You have the choice either to fulfill it or to pay an expiation for breaking an oath (kaffarat yamin), which is feeding ten poor people, or clothing them, or freeing a slave. If you cannot find any of these, then fasting for three days.