Your trip is really a bit tight, but if you don't mind driving a lot you
can do it all on highways.
Munich to Salzburg is about one hour's drive, so no sweat.
>From Salzburg to Venice you have to drive through Innsbruck, cross the
border into Italy at Brennero then drive down to Verona and then to Venice.
Not a short trip, but I guess 4 hours at 120kmh (or 75 mph) could be
enough. The road back is the same. If you like you could get a better look
of Austria driving from Salzburg southeast to Villach and then to Udine and
Venice. It's longer, about 6 hours, but you'd cross the Alps at very
scenic places and see some of Austria's pleasant countryside. You can then
go back on the first route.
3 days in Venice will be enough for getting a taste of it; remember you
can't drive into the city (it's on water!) and parking can be expensive,
from 5$/day on the mainland in Mestre to 15$/day at Tronchetto (right at
the end of the bridge to Venice) till the outrageous tariffs of central
garage in Piazzale Roma.