Seeking Guidance - Southern Bavarian Castles and Zugspitze in 3 days?

It's possible. And yes, have night stays in Füssen in any case, the ride from Munich and back is a bit tedious.

Will you have a car or will you rely on public transportation?
We recently spent 4 nights in that area- and a car makes it all so much easier!

We opted for 2 nights in Fussen then 2 nights in Garmisch, dropped our car in Garmisch and took train from there to Munich. (IIRC there are fewer car rental agencies in Fussen and the train ride to Munich was longer than from Garmisch)

However if I had to pick just one location out of those 2 -- I'd go with Fussen.

Are you checking out of you Munich hotel on day 4, when you leave for Berchtesgaden? If so, I suggest that on day 8, when you go back to Munich, you go to Freising and spend the night there. It's somewhat easier to get to the airport from Freising. There is a bus from the Freising Bahnhof; leaves every 20 minutes and the trip takes about 20 minutes. The world's oldest brewery, Weihstephen, is in Freising. Here is a list of accommodations in Freising.

I'd say the odd place is Berchtesgaden, only because it's in somewhat opposite direction of Fuessen. You'll travel 2 or more hours to Berchtesgaden. Then wake up and go back to Munich (2 hour trip) and then on to Fuessen, which is another 2 hours. Not the worst, but I'd maybe add the Berchtesgaden night to Fuessen or make Mittenwald the one night stay.

Paul

If you don't have a car and are just planning to visit the Zugspitze as a day trip, it's much easier to do it from Munich than it is from Füssen.

As of now, we plan to rent a car when we leave Munich and keep it through Zugspitze - it seems we'd have more options of where to go if we are driving.