It's called Dental paresthesia: Nerve damage as a complication of wisdom tooth extraction or dental injection.
You can read about the particulars on http://www.animated-teeth.com/wisdom_teeth/t7-wisdom-tooth-paresthesia.htm
Sorry to hear this is bothering you for almost 3 years. Apparently you can either damage or sever a nerve. and as it's been such a long time, it could be the latter. I think you should seek out a new dentist. It is unreasonable that she didn't tell you of this dental complication and i wouldn't be able to trust her for not taking the time to fill you in, as you are the one suffering and should be able to rely upon her knowledge (she must know this) . She get's paid enough to treat you better.
Yes I wouldn't think it would be normal to have numbness three years later. There are many nerves that could have been affected. I would get a second opion from a different dentist . The one that did the original work may not want to admit fault, even though if something like that did happen it of course would have been an accident. It does happen though. Usually numbness subsides within a few hours after dental surgery depending on what they used to numb you,