Amalfi Coast

At this late date, you will be hard pressed to find affordable options. Perhaps ask your cruise line if they have an add- on package. I'm assuming you don't have airline tickets that need to be changed.
PERSONALLY, I wouldn't go to the AC at this time of year, due to excessive heat and crowding.
And, three nights would be tough, especially if you have to be at an airport the night before a flight.
Hope your tour is going well!

So, you are currently in Italy? Maybe more time in Tuscany? I've heard there are beaches along the coast of Tuscany that only locals visit?

If you want to give Amalfi a try and can't find a place to stay, you might try seeing if you can get a place in Salerno. Salerno is a fun city, and just a short bus ride to Amalfi Coast. Hotel Plaza is right by the train/bus station and right on the main pedestrian seat.

I feel that your original post is unclear. Are you currently ON a tour of Italy visiting R, F, & V; or are you just booked on one in the future. If so, when? The train from Venice to Naples would take over 7+ hours and require changes. (Due to current works in Florence). Or youcould take a direct flight to Naples. Then you need to get to your accommodations on the AC, by hired car, or train, depending on where you will stay. Any plans to do this in the near future are difficult- finding accommodations at this late date is fraught to say the least.

Welcome to the forum Rose

It is impossible to offer solid advice with so little to go on. Your post is unclear.

Are you ON a tour now? If not, when is this tour?
Where does the tour end?

Do you have flights home already scheduled and from where?

You must be in your departure city the night before your departure flight (IOW you cannot stay on the AC night before or you risk missing your flight)

If you are looking to visit AC in the next few days or weeks your chances of finding any accommodations there are very very slim.
Visiting the AC at any time requires a good bit of research- especially when it comes to understanding transportation in the region.

Rose - Do you mean that you have booked a Rick Steves tour to Rome, Florence and Venice - usually called "VFR"? This is a forum for folks who are not employed by or agents for Rick Steves Tours. I'm guessing that the information you received from RS when you booked includes a phone number or email contact for the company.

The Amalfi Coast is amazing, I hope you can find what you're looking for.

I’d keep it very simple for just 3 nights and pick one base, probably Sorrento or Salerno, instead of changing hotels. If your tour ends in Rome, the easiest add on is usually the high speed train to Naples and then a transfer from there, and once you’re on the coast I’d use ferries between towns because it’s a lot less stressful than buses or driving. Travelmar has lots of local coast connections, and Ferryhopper is handy for comparing Amalfi and Capri routes. A Capri day trip is also very doable if you want one extra splurge day. I’d look at trenitalia.com/en.html, travelmar.it/en/timetable and ferryhopper.com/en/ferries/italy/amalfi.

https://www.amalficoastdaytours.com/tours-from-rome-to-amalfi-coast/
https://www.tripadvisor.com/AttractionProductReview-g187791-d11487388-Pompeii_Amalfi_Coast_and_Positano_Day_Trip_from_Rome-Rome_Lazio.html

Rose, it would help us to help you, if you give us more information.