I have worked here for three years. ( present perfect ) I started working here three years ago but I do not work here anymore. It expresses an action started in the past and ends in the present/before the moment of speaking.
No - the person could still be employed there.
The difference between the two tenses is, that in the first, the emphasis is being placed on the period of time over which the person has worked, whilst in the second sentence, the emphasis is on the activity, (this is the place, here) where I work - in the past, now, and for the (undefined/indefinite) future.
I have been working here for 3 years, and in all that time, I have not received any increase in pay.
I have worked here (now) for 3 years. Before that, I worked for Smith & Co. People tell me I should leave and find a better job, but I'm happy here.