St Goar is beautiful. Rudesheim is rather touristy especially in summer, but worth a short visit. I don't know how much interest the statue of Germania would hold for non-Germans, but the view is amazing from up there regardless.
It might be worth taking one of the Rhein cruises which can be in a variety of lengths. That way you can see many of the sites from the water, with not too much effort.
Three days to me sounds maybe one day too long, but it is difficult for me to say, having seen that area so much.
If I had to choose, I would set Rothenburg o.d Tauber aside as it has become extremely touristy in summer. A day in Munich and then two in Wiesbaden would maybe be better. A day in Munich will allow you a leisurely stroll around the pedestrian zone, the market, etc (or a city tour) and allow you to see the main sights. It would also break up the travel a little.
Garmisch-Partenkirchen to Munich is about 45-60 minutes driving time.