My guess is that renting a car is the least expensive option for three persons. You'd either pay for renting the car on the two separate days - to port form SSR and to MCO from the port. Or, pay for rental for the entire time of your cruise and the parking of it at the airport.
A towncar service for roundtrip from SSR to port and back to MCO would probably be in the area of $170.00. This is just a guess based on information I've seen on various town car company websites. I know it costs more from WDW to the port than from the port to MCO when broken down each way.
With three persons, if I didn't want to drive, I'd go with the towncar service myself.