If I have been running a fever for 3 days do I need to go to the doctor?

Fever. Temperature over 102 degrees should be investigated. The function of fever in the body is to provide an inhospitable environment for bacteria and dilate blood vessels, letting more white blood cells get to the site of the infection. Certain white blood cells are also activated by the higher temperature. Furthermore, your metabolism increases, helping the body combat infection. See your doctor!
Fever. Any fever that is higher than 101 needs further evaluation. Some fever even lower than that would also need further evaluation and depends on your medical history and immunocompromised state.