Your pro who did a calc and estimated 3.0 tons may actually be agreeing with you, I would show him my Hvac-calc numbers and ask him to discuss it with you. It would not hurt to post the actual sensible and latent BTUH numbers you got from the Hvac-Calc model. Pros on this board have been very helpful in interpreting these numbers, so you can understand what they really mean.
One other issue to worry about, is matching up the ductwork with the AC airflow required. That will be less of a problem if you go with the smaller AC. Your existing duct system has the capacity only for a certain amount of airflow, I hope it is enough for your actual BTUH needs. Asking a pro is the only responsible way to go, but I would also tell him I want to receive a measurement of ESP (external static pressure, I think you can call it back-pressure) in writing, to see how close you are to the limits of your air handler. This is only my opinion but cannot see anything wrong with getting this information.
Hope this helps -- Pstu