Is it normal for a migraine to last longer than 3 days & no relief from OTC meds?

Go to an ER ? Go to urgent care or emergency room so they can treat you with intravenous medicines. Migraines may last several days but they are very stressful.
It can happen. Migraines lasting days can happen. However, you must be under the care of a neurologist and rule out other serious problems.