Philip, as the others have indicated, it's highly unlikely you'd be able to visit all three locations and still make it back to the ship in time.
While it may be theoretically feasible to visit all three locations, the reality is different. I visited Capri in May and spent the whole day there. There is often somewhat of a wait for the Funicular up to the main part of town, and I found that it's worse coming down (I ended up taking a Taxi so I wouldn't miss the last boat of the day). Plan to visit Capri and Annacapri on a future visit (and bring LOTS of money - it's expensive!).
Your best bet would probably be to spend a few hours in Sorrento and Pompeii. Be sure to wear your Money Belt on the Circumvesuviana, and most importantly allow enough time for the trip to Sorrento, to Pompeii and back to Naples (include "waiting time" for the trains). Check Rick's books for details on the location of the Circumvesuviana station in Naples, costs and other details.
Happy travels!