How can I help a friend who's been to rehab 3 times and still hasn't help her drug addiction

3 treatments. One of the most common causes of drug addiction treatment failure is treating the co existing disease such as bipolar disorder, anxiety disorders etc. The latter exist in greater than 60 percent with addiction and the second most common cause of failure is that patients stop medications helping them contain their anxiety. Treatment compliance is a big challenge for recovering patients.
Good question. Not an easy answer, depending on your relationship with her, it may take "tough love" as there are consequences for her behavior. I would stronly encourage you to attend al-anon, which is a program for friends and loved ones that are affected by someone else's addiction. Learn more about the disease of addiction and how it affects you. Not enough room to write more, good luck.
Keep encouraging. Unfortunately addiction is often a chronic recurring brain disease. Continue encouraging your friend to keep fighting the disease. Life style changes are usually necessary. 12 step meetings, daily exercise, stress management, individual or group therapy can be helpful. Medications may be needed for other psych disorders. Addiction is treatable but staying abstinent is tough work.