What would cause extreme pain in the ear? It's been on-going for 3 days now.

Infection. An ear infection could be one cause for severe ear pain. This could be either a swimmer's ear or a middle ear infection. There are several other potential causes that could be evaluated depending on other symptoms you may have. After three days without improvement, this should be evaluated promptly.
Ear pain. Usually an infection - pain for three days should be checked.