What respectable horse owner does not pull a coggins in 3 years? Even if the horse hasn't left the property in that time. I don't care if my horse hasn't been anywhere in 5 years, I still pull a coggins every year.
If it has been 3 years for a coggins consider what else hasn't been done for the horse. What about the horse's teeth? When was the last time that was done? What about other vaccinations? You said the horse needs a rabies shot, so I would think its safe to assume the horse needs others as well. You may want to reconsider this purchase. If you really want this horse I would ask the owner to get a coggins and get the horse up to date on all of its vaccinations. You should not have to pay a dime. These type of things are standard when buying a horse! If she isn't willing to do it then walk away. You'll find another horse.