First, you can find a spa that will utilize the wiring that is there. Sounds like a Hot Springs circuit set up. Consult with your local Hot Springs dealer.
Secondly, wire size dictates what you can do with the current wiring, and your best advice will come from an electrician.
Shop around, look for what is out there, understand the electrical requirements and compare it to Hot Springs. Depending on the advice of your electrician, you may be able to "pull" new 8ga. or 6ga wire for a 50 or 60 amp circuit.
The electrician can advise you what your options are electrically, then you can focus your interest on what that advise dictates. you may be limited, then again, you may have all of the options available.
NOW, how are you going to get it to the 3rd. floor?