I've had a headache for 3 days straight. The pain is in the right side of my head. Is this serious?

Yes. Any new or changing ha is serious. There may be many causes and some more serious than others. It may be a simple casue, but you may not be trained to know the difference. Depending on age, medical diagnoses, medications, activities and risks, you could have any number of causes. Temporal arteritis, aneurysm, tumor, infection, neuralgia, and more. See a doctor and i would say very soon.

Exam and history. Too vague a question without having a thorough history and an examination. Too many variables when attempting to determine the cause of a headache. The cause, severity and location of the headache are very important.