Has the system been empty for three years? If so, the receiver drier is no good. Why don't you fill the system (if it will hold refrigerant long enough) and function test it? That way you'll know if the compressor or expansion valve are working. If you've been topping it off periodically and haven't let it run dry you should be able to re-use the drier. Keep in mind that most people would recommend replacing the drier anytime the system is opened up, but if you're looking to save money I don't think re-using it is that bad an idea. Whatever you decide, make sure to add some UV dye to the system after you fix it. That will make finding any future leaks a lot easier.