In 3 weeks is this itinerary doable? I've given rough estimates of time in each. Combination of buses and plains. Possibly skipping Iguazu as will keep it for a trip to Brazil.
Buenos Aires 4 days
Salta 5 days
Mendoza 3 days
Bariloche 3 days
El Calafete + El Chalten 3 days
I have a couple more days spare I could add to one of these destinations.