My first thought is all those itineraries have too many places that are too far apart. You will spend the vast majority of your trip in airports, in train stations, or in limbo with your backpacks waiting to check-in to airbnbs or hostels. Things in Europe are close together but not that close!
Keep in mind that three nights in a place is essentially one full day with two partial/travel days on either end of it. Travel days tend to be lost days in terms of really doing or seeing much, particularly if you fly, since airports are often a long distance from the city centres and require advanced check-ins. And with so many transfers you'll be at risk of something like an unexpected flight delay or train strike throwing your whole carefully planned schedule into disarray.
Honestly, I know it's hard but I would choose maybe two major regions (say London/Paris and Rome) as "bases", staying at each for a longer time and then doing shorter side-trips from those to see some smaller towns/villages. I think you'll see more and be able to appreciate more.
There's no way you will see every part of Europe in 3 weeks so it's best not to try -- you can always come back!