I know Vienna quite well, have been to Salzburg several times and Munich is home for me. In any normal year, all three are a good choice around Christmas (although Munich is nicer in summer or autumn, IMO), but things just won't be the same due to Covid.
Infections and hospital admissons have exploded here in Bavaria and we're back to mandatory FFP2 / N95 masks in all shops, museums, on any public transport and in restaurants and theatres when not at your table / in your seat. Not great if you're out and about all day and unlikely to change until after Christmas. I don't see a complete lockdown on the cards, but stricter rules seem a very real possibility within the next few weeks.
The Christmas markets will apparently be spread out over larger areas of the city this year to avoid large crowds forming. Also less stalls allowed to sell alcohol. This may or may not take away form the atmosphere somewhat. Some of the (especially lovely) smaller Christmas markets near Munich have been cancelled altogether.
I live in a very central location with lots of bars and restaurants and a busy nightlife. While clubs were finally allowed to re-open about 6 weeks ago and lots of people are going out, it definitely doesn't feel like we're back to normal. I've noticed reduced opening hours in many restaurants and there's generally still less on offer in terms of concerts, exhibitions, theatre, sightseeing stuff, etc. Many things you could normally do spontaneously require pre-booking now. A lot of entertainment stuff also seems to sell out weeks in advance due to reduced capacity.
I know it's not what you want to hear, but unfortunately I have agree with CupcakeTowers...probably better to wait until next year.