All 3 totally worth it- I would say Prague is the cheapest and Rome the most expensive, Prague I would recommend doing the Sandemanns free walking tour and then maybe one of their other paid ones: so worth it. Also anonymous bar, and go up any towers you can as the roofs are so pretty. Go up to the castle in golden hour!! And don’t buy the ham in the main town square, the price is per 100g and they’ll force a kg on you
Dubrovnik... maybe the shortest trip of the 3, we did everything we wanted to do (walked all round the walls, went up in the cable car, explored the centre) in 6 hours from a cruise ship. I’m sure you could find more to do but it won’t be as jam packed as the others, again might be what you want.
I won’t bother telling you about Rome, obviously the grandest of the 3, all walkable in the centre. Prebook a tour for the coliseum to jump the queues, we used viator. And like I said it’s the most expensive of the three you mentioned by far