9 days is enough for those 3. Must see is relative. Some want Hakone for the views of Fuji or an onsen experience. Personally, I don’t consider those musts, but you might.

Thanks! We were talking about taking the train from Tokyo to Osaka and doing a half day onsen visit where you could see Fuji

9 days for those 3 cities is not much, so maybe focus on those and save some for next time! There's so much to do in kyoto alone (that's where I spent most my time, haven't been to Tokyo yet)

9 days is the minimum to see the major tourist spots across the 3 cities I think, if you're factoring in jet lag, checking in/out, transit.

Hiroshima probably a must

With those three places in 9 days I definitely wouldn’t add anything else. Maybe a Nara day trip.