3 weeks in Switzerland, Austria & Germany

You might start by looking at the Rick Steves Germany, Austria, Switzerland tour in 14 days and add some more time to that.
https://www.ricksteves.com/tours/germany/germany-austria-switzerland-tour

That suggests flying into Frankfurt and out of Vienna, but you will have to figure out the best flights based on where you are coming from.

thank you. you obviously know these countries well. Could you help me with a 3 week itinerary.

What an exciting trip! I love all three countries and July should be a wonderful time of year to enjoy the beautiful scenery in all of them.

Do you have any particular interests that might influence what you'd like to do on this trip? Hiking? Museums and galleries? History? Spas?

I'd be happy to share some ideas but it would be nice to tailor them to what you like to do!

"Could you help me with a 3 week itinerary."

Welcome to the forum, btvolner. The best way to get help on this forum is to first develop a preliminary list of destinations and/or activities that interest you most. A that point we can probably provide some helpful guidance. Providing you with a specific 3-week itinerary would be inappropriate and possibly a big waste of time. You've traveled a lot already, so you know your own travel preferences and interests much better than we do. All we know are the countries that you're targeting, that you want to go by train, and our own personal preferences. These are 3 great countries for train travel, but trains go almost everywhere in this part of Europe, so this preference doesn't narrow the options very much. "Which cities & sites are a must see" depends on your own "musts." Some guidebooks from your local library or bookstore (don't rely on Rick Steves alone) could help you make some informed choices, but don't rely on their "musts" either.

We are also two seniors who travel by rail and bus. Suggest flying into Vienna and working your way west. Short train trip from Vienna to Krems, along Danube. We stayed a few miles away in lovely Durnstein. The Wachau Valley is beautiful. You can bus or ferry to Melk Abbey and back. Train from Krems to Salzburg, lots to see and do. Sound of Music tour sounds hokey but it was loads of fun, with beautiful sights. You could tour the Eagle's Nest if the weather is good. Easy train ride from Salzburg to Munich, suggest buying tickets from DB instead of Austrian rail in Salzburg cheaper with some neat options. Lots to see in Munich. We then trained south to Garmish- Particshkirten, great hike up an incredible gorge or you could gondola up top to the Zugspitze, also visited the sight of the 1936 winter Olympics. A short train ride from there is a neat village, Mittenwald worth spending at least a day. You could work your way over to Switzerland from there. suggest spending time in the Berner-Oberland, Interlakken, the Eiger, etc.. See what you want by train in Switzerland and fly home out of Zurich. this would make a convenient route with out having to double back anywhere. What a great trip, Enjoy! Hank, Novato, CA

I would look at flying into Vienna and out of Zurich. Having no idea what your preferences are, I would recommend Vienna (with some day trips to Wachau Valley, possible day trip to Bratislava) to Salzburg (include various daytrips like Halstat, Eagles Nest, etc.) to Munich (including day trips per your interests) Luzern [including day trips up one of the mountains (Rigi, Titlis, Pilatus), to Bern, possibly Basel] to Berner Oberland(at least 4 nights, stay in Lauterbrunnen) After research, you could assign the number of days to each. This could all be done by train.
Now people with different interests and varying travel style will pick this apart which is why I hesitate to suggest any specific itinerary, in addition to the fact that we don't know your preferences. Travel, while well worth it, is expensive. I would do tons of research and choose places and lengths of stay based on my interests and how I like to travel.