Five days is enough for Split, one Dalmatian island and Dubrovnik only when the route stays linear. Begin beside Split's harbour, cross to Hvar Town, then continue south to Dubrovnik. Each move doubles as a change of scenery: Roman courtyards become an island waterfront, then Dubrovnik's limestone walls close the trip. Flying into Split and home from Dubrovnik protects almost a full day compared with returning to the first airport.
The price of that efficiency is depth. This is not a beach week and it is not a tour of Croatia as a whole. Split earns two nights because arrival delays, summer traffic from the airport and a timed interior can otherwise consume its only useful morning. Hvar receives one night, enough for the fortress, harbour and a swim but not for Stari Grad, vineyards and remote coves together. Dubrovnik also gets one night, so the city walls must sit at the cool edge of the day. Travellers who dislike packing twice in four mornings should choose Split plus Hvar or Split plus Dubrovnik, not force all three names into the calendar.